[steps] Expand legacy command/path local functions to a single build step

[sswrk]

Why

Makes the moved functions actually run. A workflow step calling a command or path function should behave the way it did in a .eas/build custom build.

Builds on #4095, which taught function.yml the single-step shape.

How

When a step references a local function whose function.yml declares command or path, the expander emits a single build step instead of expanding a step group, reusing the same BuildFunction.createBuildStepFromFunctionCall machinery that runs these functions in custom builds today. Because that method already accepts a composite function scope, calls work in job steps, in hooks, and from inside a composite function.

• New utils/legacyFunction.ts maps the parsed config onto a BuildFunction. Legacy semantics are preserved: inputs and outputs are required unless declared otherwise (the composite shape keeps its required ?? false default for inputs), and supported_platforms is enforced by the existing step-level platform validation.
• The function-level shell field now takes effect. In .eas/build configs it was parsed and stored but never applied to the step.
• CompositeFunctionExpander.expandCompositeFunctionStep becomes expandLocalFunctionStep and returns flattened BuildStep[], dispatching on the shape, so CompositeBuildStep no longer leaks to call sites. A single-step function keeps the caller's own id (no caller__inner prefix) and takes the call-site if: as its own condition; there is no expansion scope to hang it on.
• The catalog builder treats single-step functions as leaves (no steps to recurse into, so no cycle risk).
• Adds the shared resolveLegacyFunctionModulePath helper used by the loaders in the next two PRs, so the module-path rewrite cannot drift between them.
• working_directory on a step that calls a local function stays rejected for both shapes; the error message is now shape-agnostic. Widens CompositeFunctionCatalog to hold either shape.

Test Plan

cd packages/steps && yarn test

New StepsConfigParser-legacy-functions-test.ts covers expansion to exactly one step with the caller's id, display-name fallbacks, with: values reaching inputs, defaults, required-input and allowed_values validation, outputs via set-output and their visibility to later steps, supported_platforms enforcement, forwarded shell, a path function producing a step that calls the module, rejected working_directory, and calls nested inside a composite function (id outer__inner, ${{ inputs.* }} into with:, outputs visible to siblings and to the composite's output templates). Extends the hooks and catalog-builder suites.

[linear-code]

ENG-25402

[sswrk]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [eas-cli] Validate legacy command/path local functions referenced from workflows #4098
• [build-tools] Load legacy command/path local functions and resolve module paths #4097
• [steps] Expand legacy command/path local functions to a single build step #4096 👈 (View in Graphite)
• [eas-build-job] Accept legacy command/path custom function shape in function.yml #4095
• [eas-cli] Validate local composite functions referenced from workflow hooks #4064
• [build-tools] Load composite function catalogs for hook steps #4063
• [steps] Support local composite functions in hook steps #4062
• [steps] Unify the default gating of steps with no if: across steps, composite scopes, and hooks #4061
• [eas-cli] Validate local composite functions in workflow files #3931
• [build-tools] Load local composite function catalog at build runtime #3930
• [steps][4/4] Resolve composite function outputs #4015
• [steps][3/4] Interpolate composite function inputs #4014
• [steps][2/4] Expand composite function references into build steps #3929 : 1 other dependent PR (#4013 )
• [steps][1/4] Add isCompositeFunctionPath and reject local composite functions in custom builds #3956
• [eas-build-job] Add local composite function schema #3928
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[codecov]

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Additional details and impacted files

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