refactor(internal/librarian): dispatch fillDefaults by language instead of type switch

[JoeWang1127]

Replace the type switch in fillDefaults that inferred the language from which Default field was non-nil with a languageDefaultFillers map keyed by the explicit language string from Config.Language.

This eliminates the implicit priority ordering between languages (e.g. Java before Rust) and the fragile dispatch based on data rather than the known language identifier.

Each language-specific fill function now guards against its default field being nil, since the type switch no longer prevents calls when the language-specific default is absent.

Motivated by #6554 (comment)

[gemini-code-assist]

Code Review

This pull request refactors the fillDefaults function in internal/librarian/library.go to use a map lookup (languageDefaultFillers) instead of a large switch statement. To prevent nil pointer dereferences, safety checks have been added to each language-specific filler function. Additionally, the unit tests in internal/librarian/library_test.go have been refactored into language-specific test functions and updated to pass the new language parameter. There are no review comments, and I have no feedback to provide.

[JoeWang1127]

Talked with @ianthehat, this change doesn't solve the design issue of config.Config: it ties the public API to a serialize form so they have to change in sync.