[steps] Unify the default gating of steps with no if: across steps, composite scopes, and hooks

[sswrk]

Why

For a step with no if:, whether the step should run was decided in 3 separate places: BuildStep.shouldExecuteStep, the composite function scope (same rule), and the hook executor (its own rule). That bypass means composite scopes never see hook defaults, which is a blocker for adding support for composite functions in hooks.

How

A single gate now decides whether any step runs, taking a single input: a bool telling whether a step with no if: should run. Composite functions called from hooks therefore inherit the hook's rule. Steps with an explicit if: are unaffected.

Behavior-preserving refactor.

Test Plan

• Existing steps/hook tests still pass.
• Added unit coverage for the run-by-default value on shouldExecuteStep /
  isActive, and hook sibling skip/continue cases for the before/after defaults.

[linear-code]

ENG-22387

[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
• [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 👈 (View in Graphite)
• [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]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (szymonswierk/eng-22387-eas-cli-validate-custom-actions-in-workflow-files@ab74d1e). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
packages/steps/src/BuildWorkflow.ts	57.15%	3 Missing ⚠️

Additional details and impacted files

@@                                             Coverage Diff                                             @@
##             szymonswierk/eng-22387-eas-cli-validate-custom-actions-in-workflow-files    #4061   +/-   ##
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  Coverage                                                                            ?   62.22%           
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  Files                                                                               ?      995           
  Lines                                                                               ?    44754           
  Branches                                                                            ?     9412           
===========================================================================================================
  Hits                                                                                ?    27845           
  Misses                                                                              ?    15462           
  Partials                                                                            ?     1447           

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[claude]

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[hSATAC]

LGTM.

[github-actions]

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