Propagate scaffolding failures to CLI exit codes

[danroth27]

Summary

Fixes #3845.

Required scaffold-step failures currently stop execution but are discarded by the scaffolder, command handler, runner, and process entry point. This can leave partial changes while reporting exit code 0.

• Return the required-step result from IScaffolder.ExecuteAsync and propagate failure as exit code 1 through the CLI.
• Preserve failures through interactive command execution, including external tools, and add exit-code-returning root handlers. Cache the execution result so the interactive validation fallback cannot report success or rerun the command.
• Identify the failed step and warn that the project or external resources may have been partially modified.
• Preserve existing skipped-step, ContinueOnError, and pre/post-execution callback behavior.
• Propagate dotnet add package failures instead of always returning success; package-installation steps are required, so stop before subsequent code-generation steps on failure.
• Return success for an existing generated static file only if its bytes match the template. Report a differing file as a conflict without overwriting it.
• Normalize negative flow results to process exit code 1 at the common runner boundary. Tool install/uninstall commands also return 1 when their manager reports failure.
• Add focused regression coverage without changing the core README.

Compatibility and coordination

IScaffolder.ExecuteAsync now returns Task<bool> and IScaffoldRunner.RunAsync returns Task<int>. Custom interface implementations need to update their signatures; existing callers can still await and ignore the result. Existing Task root-handler overloads remain available.

The runner/process changes overlap with the still-open #3806. This PR additionally fixes the earlier loss of required-step failures and interactive result propagation; coordinate those overlapping changes when merging.

This shared fix also addresses the false-success report for missing client-project references in the Blazor Identity updates in #3836 (for #3797): ValidateIdentityStep already returns false, but its result was discarded. The exact combined Identity-branch repro has not been run.

Missing tool-internal code-modification recipes can still cause some preparers to skip required work. That broader packaging/layout failure path is deferred to a focused follow-up; it is not the same as missing application configuration.

Validation

• Reproduced a required step returning false while the command and an Entra CLI validation failure returned exit code 0 before the fix.
• 126 targeted command-line, interactive-flow, Aspire-step, package, and static-file tests pass. A real failed dotnet add package now propagates through a scaffolder to exit code 1; identical existing static files succeed without a write and modified files fail without being overwritten.
• The built-in flow tests use production interactive mode (nonInteractive: false), verify the fallback returns the actual command failure, and assert exactly one execution. A private Windows console buffer avoids unrelated headless-console errors; the original handles are restored afterward.
• Negative action results (-1 and int.MinValue) normalize to 1 at the runner boundary; positive action codes continue to pass through.
• Built dotnet-scaffold for .NET 8, 9, 10, and 11 with no warnings or errors.
• Four Blazor Identity CLI integrations (net8.0–net11.0) pass with the package-failure and static-file behavior changes.
• Invoked the built tool in separate processes on all four target frameworks: Entra validation failure returns 1, an unknown option returns 1, and help returns 0.
• Created a fresh MVC project and ran razorview-empty using the .NET 11 tool; verified exit code 0 and the generated Views/ExitCodeSuccess.cshtml file, then removed the temporary project.

dotnet test .\test\dotnet-scaffolding\dotnet-scaffold.Tests\dotnet-scaffold.Tests.csproj --filter 'FullyQualifiedName~CommandLine.|FullyQualifiedName~Interactive.Flow.|FullyQualifiedName~Aspire.ScaffoldSteps.|FullyQualifiedName~AspNet.ScaffoldSteps.AddFileStepTests|FullyQualifiedName~AspNet.ScaffoldSteps.WrappedAddPackagesStepTests|FullyQualifiedName~Scaffold_BlazorIdentity_' -p:BuildTfm=net11.0 -m:1 -nr:false
dotnet build .\src\dotnet-scaffolding\dotnet-scaffold\dotnet-scaffold.csproj -m:1 -nr:false

No tenant-backed provisioning was performed. Failure reporting does not introduce rollback of earlier changes.

[Copilot]

Copilot review overview

🟡 Changes recommended

The required-step failure path skips the established post-execution callback.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 1

Open (1)

• Preserve post-execution callback before returning on failure · New

What changed in this PR

This PR propagates required scaffolding failures through interactive execution and the CLI so failed operations return nonzero exit codes.

Changes:

• Returns execution results from scaffolder and runner APIs.
• Propagates built-in and external command failures.
• Adds failure diagnostics and regression coverage.

Review findings:

• Moderate (2 votes): Scaffolder.cs:73 returns before RunPostExecute, changing the callback contract. Defer the failure return until after the post-execution callback.
• Nit (1 vote): ScaffolderTests.cs:58 codifies that regression; keep post2 unconditional and gate only step-3 events on success.

File	Description
test/​dotnet-scaffolding/​dotnet-scaffold.Tests/​Interactive/​Flow/​CommandExecuteFlowStepTests.cs	Tests interactive failure propagation.
test/​dotnet-scaffolding/​dotnet-scaffold.Tests/​CommandLine/​ScaffoldExitCodeTests.cs	Tests process exit codes and diagnostics.
test/​dotnet-scaffolding/​dotnet-scaffold.Tests/​CommandLine/​ScaffolderTests.cs	Tests step ordering and callback behavior.
test/​dotnet-scaffolding/​dotnet-scaffold.Tests/​CommandLine/​FullHelpPrinterTests.cs	Updates the test scaffolder contract.
test/​dotnet-scaffolding/​dotnet-scaffold.Tests/​Aspire/​ScaffoldSteps/​WrappedAddPackagesStepTests.cs	Updates the test scaffolder contract.
test/​dotnet-scaffolding/​dotnet-scaffold.Tests/​Aspire/​ScaffoldSteps/​ValidateOptionsStepTests.cs	Updates the test scaffolder contract.
test/​dotnet-scaffolding/​dotnet-scaffold.Tests/​Aspire/​ScaffoldSteps/​AddAspireConnectionStringStepTests.cs	Updates the test scaffolder contract.
test/​dotnet-scaffolding/​dotnet-scaffold.Tests/​Aspire/​ScaffoldSteps/​AddAspireCodeChangeStepTests.cs	Updates the test scaffolder contract.
src/​dotnet-scaffolding/​Microsoft.DotNet.Scaffolding.Core/​Scaffolders/​Scaffolder.cs	Returns required-step results and logs failures.
src/​dotnet-scaffolding/​Microsoft.DotNet.Scaffolding.Core/​Scaffolders/​IScaffolder.cs	Exposes Boolean execution results.
src/​dotnet-scaffolding/​Microsoft.DotNet.Scaffolding.Core/​CommandLine/​CommandLineExtensions.cs	Maps scaffold failures to exit code 1.
src/​dotnet-scaffolding/​Microsoft.DotNet.Scaffolding.Core/​Builder/​ScaffoldRunnerBuilder.cs	Adds exit-code-returning root handlers.
src/​dotnet-scaffolding/​Microsoft.DotNet.Scaffolding.Core/​Builder/​ScaffoldRunner.cs	Returns command invocation exit codes.
src/​dotnet-scaffolding/​Microsoft.DotNet.Scaffolding.Core/​Builder/​ScaffoldBuilder.cs	Supplies failure logging.
src/​dotnet-scaffolding/​Microsoft.DotNet.Scaffolding.Core/​Builder/​IScaffoldRunnerBuilder.cs	Declares result-returning handlers.
src/​dotnet-scaffolding/​Microsoft.DotNet.Scaffolding.Core/​Builder/​IScaffoldRunner.cs	Declares exit-code-returning execution.
src/​dotnet-scaffolding/​dotnet-scaffold/​Program.cs	Returns the runner’s process exit code.
src/​dotnet-scaffolding/​dotnet-scaffold/​Interactive/​Flow/​Steps/​CommandExecuteFlowStep.cs	Propagates interactive command failures.

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[Copilot]

Copilot review overview

🟡 Changes recommended

A critical issue still allows built-in failures to return exit code 0 in the normal interactive path.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 1

Open (1)

• Interactive scaffold ignores validation failures and reports success · New

Resolved since last review (1)

• Preserve post-execution callback before returning on failure

[Copilot]

Copilot review overview

🟡 Changes recommended

Critical exit-code handling and moderate passkey rerun behavior remain unresolved.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 1

Open (1)

• Interactive exceptions can return success exit status · New

Resolved since last review (1)

• Interactive scaffold ignores validation failures and reports success

[Copilot]

Copilot review overview

🟢 Approval recommended

Failure propagation is consistent across execution paths and covered by focused regression tests.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Interactive exceptions can return success exit status