[eas-build-job] Add local composite function schema

[sswrk]

Why

First step toward local composite functions for EAS Workflows: reusable step groups referenced via uses: in .eas/workflows/*.yml.

This PR adds the shared contract in @expo/eas-build-job so all consumers agree on composite function config shape.

How

Introduce the composite function config schema in @expo/eas-build-job, the single source of truth for the shape of a function.yml file. No runtime behavior changes yet.

Added:

• CompositeFunctionConfigZ / CompositeFunctionConfig: Zod schema and type for function.yml (inputs, outputs, runs.steps)
• CompositeFunctionCatalog: map of normalized path → config

Test Plan

Added unit tests for CompositeFunctionConfigZ.

[linear-code]

ENG-22387

[sswrk]

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

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.13%. Comparing base (a48ad7a) to head (b4397b8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3928      +/-   ##
==========================================
+ Coverage   61.12%   61.13%   +0.01%     
==========================================
  Files         970      971       +1     
  Lines       43423    43431       +8     
  Branches     9135     9135              
==========================================
+ Hits        26538    26546       +8     
  Misses      15439    15439              
  Partials     1446     1446              

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🤖 AI code review

Decision: Approve

All specialist reviewers (correctness, consistency, security) returned no findings. The PR introduces only a Zod schema and TypeScript type for composite function configuration with no runtime behavior changes.

No findings.

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This review is advisory — it never blocks a merge and never auto-approves.

[github-actions]

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