test(internal/librarian/java): use sentinel errors with errors.Is for error validation

[perashanid]

Address review feedback by replacing string-based error checking with sentinel errors following the project's Go style guide.

Changes:

• Add errInvalidDistributionName and errAPIConfigNotFound sentinel errors (unexported)
• Wrap sentinel errors in collectModules error returns using %w
• Update TestCollectModules_Error to use errors.Is instead of strings.Contains
• Remove unused strings import from test file

This makes error checking more robust and maintainable by avoiding fragile string comparisons that can break when error messages change.

Fixes #4962

[gemini-code-assist]

Code Review

This pull request refactors error handling in the Java librarian by introducing sentinel errors for invalid distribution names and missing API configurations, updating the code to wrap these errors and the tests to verify them using errors.Is. A critical issue was identified in pom_test.go where a merge conflict marker and duplicate code were left in the file, which results in invalid syntax and must be resolved.

[perashanid]

@julieqiu reopened the pr here again , in the earlier i think i messed up with the rebase

here is the orginal pr with rebase problem #4965

[noahdietz]

This isn't necessary - errors.Is will handle if err is nil and we'd get collectModules() error = nil, want ...".

Suggested change

	if err == nil {
	t.Fatal("collectModules() error = nil, want non-nil")
	}

[julieqiu]

@perashanid - thanks for the contributions. Our internal development is moving so quickly at this point that external PRs are likely to fall out of sync almost immediately, making it difficult for us to review and integrate external contributions effectively. For that reason, we’ve decided to keep development internal for the time being. I'm going to close this out to save you any further effort on updates. We appreciate the initiative regardless!