fix(internal/librarian/nodejs): skip writeRepoMetadata when file is in keep list

[julieqiu]

writeRepoMetadata unconditionally overwrites .repo-metadata.json after combine-library runs, even when the file is in the library's keep list. The keep/restore logic correctly preserves the file, but writeRepoMetadata then replaces it with a generated version that differs in formatting and content.

This checks for .repo-metadata.json in the keep list before calling writeRepoMetadata, so that existing repo metadata files are preserved during generation.

For #4743

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.25%. Comparing base (ec072a6) to head (2285958).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/librarian/nodejs/generate.go	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4791   +/-   ##
=======================================
  Coverage   81.25%   81.25%           
=======================================
  Files         129      129           
  Lines       10875    10876    +1     
=======================================
+ Hits         8836     8837    +1     
  Misses       1396     1396           
  Partials      643      643           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gemini-code-assist]

Code Review

This pull request correctly prevents writeRepoMetadata from overwriting .repo-metadata.json when it is present in the library's keep list. The change is implemented by adding a check before the function call and is accompanied by a new test case that verifies the intended behavior. The implementation is clear, correct, and well-tested.