[steps][1/4] Add isCompositeFunctionPath and reject local composite functions in custom builds

[sswrk]

Why

Local composite functions use uses: ./... paths. Custom build configs (.eas/build/*.yml) must reject these early with a clear error.

How

• Adds isCompositeFunctionPath in localCompositeFunctions.ts
• Rejects local composite function paths in validateAllFunctionsExist for custom builds

Test Plan

Added unit tests.

[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 👈 (View in Graphite)
• [eas-build-job] Add local composite function schema #3928
• main

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.13%. Comparing base (197edf5) to head (1badf0f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3956      +/-   ##
==========================================
+ Coverage   61.13%   61.13%   +0.01%     
==========================================
  Files         971      972       +1     
  Lines       43431    43439       +8     
  Branches     9135     9137       +2     
==========================================
+ Hits        26546    26554       +8     
  Misses      15439    15439              
  Partials     1446     1446              

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[claude]

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[sjchmiela]

preliminary review -- i would also maybe recommend moving the here-unused functions to the pull request which starts using them cause it's harder to understand the context like this

[github-actions]

🤖 AI code review

Decision: Approve with comments

⚠️ The AI review could not complete: every review pass failed or timed out, so these changes were effectively NOT reviewed. Treat this as "no review", not "looks good".

⏱️ Coverage note: coverage is partial — some review passes did not
finish (timed out or failed), so issues may exist in areas not fully reviewed:

• The consistency review failed to run; those changes were not reviewed.
• The correctness review failed to run; those changes were not reviewed.
• The security review failed to run; those changes were not reviewed.

No findings.

---

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.