[steps][2/4] Expand composite function references into build steps

[sswrk]

Why

EAS workflows need reusable local composite functions: a step like uses: ./.eas/functions/setup should run the steps defined in function.yml, not be treated as a built-in function.

This PR adds the structural layer for that: turning a composite function call into concrete workflow steps with correct ids, scoping, and control flow. Input interpolation, outputs, and loading function.yml from disk are handled in separate PRs.

How

When StepsConfigParser sees a local uses: path (./ or ../), CompositeFunctionExpander replaces the call with a flat list of inner steps:

• ID namespacing: inner step read under caller setup becomes setup__read
• Each expanded step carries a BuildStepCompositeFunctionScope so ${{ steps.* }} and ${{ inputs.* }} resolve against composite-function-local names
• Nested local composite functions are expanded recursively with cycle and depth checks
• Local composite functions are rejected in hooks

Test Plan

Added expansion unit tests covering nesting, cycles, depth limits, and id collisions.

[linear-code]

ENG-22387

[sswrk]

• [steps][2/4] Expand composite function references into build steps #3929 : 2 dependent PRs (#4013 , #4014 ) 👈 (View in Graphite)
• [steps][1/4] Add isCompositeFunctionPath and reject local composite functions in custom builds #3956
• [eas-build-job] Add local composite function schema #3928
• main

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[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.29%. Comparing base (fcea341) to head (8c0a57d).

Files with missing lines	Patch %	Lines
...kages/steps/src/BuildStepCompositeFunctionScope.ts	63.64%	12 Missing ⚠️
packages/steps/src/CompositeBuildStep.ts	56.53%	10 Missing ⚠️
packages/steps/src/BuildStep.ts	94.24%	2 Missing and 1 partial ⚠️
packages/steps/src/BuildStepInput.ts	60.00%	1 Missing and 1 partial ⚠️
packages/steps/src/CompositeFunctionExpander.ts	97.81%	2 Missing ⚠️
packages/steps/src/StepsConfigParser.ts	95.66%	1 Missing ⚠️
...tepsConfigParser-composite-functions-test-utils.ts	95.00%	1 Missing ⚠️
...ackages/steps/src/utils/localCompositeFunctions.ts	94.12%	1 Missing ⚠️
packages/steps/src/utils/step.ts	92.31%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3929      +/-   ##
==========================================
+ Coverage   61.13%   61.29%   +0.16%     
==========================================
  Files         972      977       +5     
  Lines       43439    43687     +248     
  Branches     9137     9186      +49     
==========================================
+ Hits        26554    26774     +220     
- Misses      15439    15465      +26     
- Partials     1446     1448       +2     

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[github-actions]

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[hSATAC]

Nice, this looks solid — expansion, namespacing, cycle/depth, the if gating all read well. Two small notes inline, nothing blocking.

[github-actions]

⏩ The changelog entry check has been skipped since the "no changelog" label is present.