refactor(internal/librarian/java): rename generateRepoMetadata and move to repometadata.go

[zhumin8]

Move logic to generate repo metadata into repometadata.go, and move it before client code generation. Moved transport rename logic to serviceconfig package.

For #4308

[gemini-code-assist]

Code Review

This pull request refactors the generation of repository metadata for Java libraries. The deriveRepoMetadata function and its associated test have been moved from internal/librarian/java/postprocess.go to internal/librarian/java/repometadata.go. The generate.go file now calls the refactored generateRepoMetadata function earlier in the process, and the original postProcessLibrary function in generate.go now returns nil as its responsibilities have been shifted. A minor typo in a comment was noted.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.49%. Comparing base (71f7d3b) to head (46976c8).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/librarian/java/repometadata.go	84.37%	5 Missing and 5 partials ⚠️
internal/librarian/java/generate.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4839      +/-   ##
==========================================
+ Coverage   81.37%   81.49%   +0.12%     
==========================================
  Files         130      130              
  Lines       10905    10917      +12     
==========================================
+ Hits         8874     8897      +23     
+ Misses       1392     1380      -12     
- Partials      639      640       +1     

☔ 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.