fix(internal/librarian/nodejs): stop renaming snippet metadata files

[julieqiu]

The copySamplesFromStaging function was renaming snippet metadata files from the generator's underscore pattern (snippet_metadata_.json) to a dot pattern (snippet_metadata..json). This caused repos to end up with both naming variants, since the old dot-pattern files were never cleaned up.

Use the generator's underscore pattern as the canonical name.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.39%. Comparing base (57af767) to head (8552dd7).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4840      +/-   ##
==========================================
- Coverage   81.40%   81.39%   -0.01%     
==========================================
  Files         130      130              
  Lines       10895    10891       -4     
==========================================
- Hits         8869     8865       -4     
  Misses       1387     1387              
  Partials      639      639              

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[gemini-code-assist]

Code Review

This pull request updates the naming convention for snippet_metadata files in the Node.js generator. The logic that previously renamed snippet_metadata_<api>.json to snippet_metadata.<api>.json has been removed from internal/librarian/nodejs/generate.go. Correspondingly, tests in internal/librarian/nodejs/generate_test.go have been adjusted to expect the snippet_metadata_<api>.json format directly, and the test verifying the absence of the old underscore-prefixed names has been removed. There are no review comments to provide feedback on.