Extract programming language resolver

What does this MR do and why?

Projects::DetectRepositoryLanguagesService mixes repository language updates with ProgrammingLanguage lookup and creation. This makes the service harder to change when adjusting language matching behavior.

This MR:

• Adds Gitlab::LanguageDetection::ProgrammingLanguageResolver.
• Moves ProgrammingLanguage lookup and creation into the resolver.
• Consumes Gitlab::LanguageDetection::DetectedLanguage objects.
• Preserves existing behavior without changing language matching.
• Removes unused name-based color and Gitaly ID helper methods.
• Adds resolver specs documenting current behavior.

This is a stacked refactor on top of !241475 (merged). It intentionally does not include the Mathematica/Wolfram language matching fix.

References

#602294 (closed) !241475 (merged)

How to set up and validate locally

1. Check out the stacked base branch:

   git switch 602294-refactor-language-detection-object

2. Check out this branch:

   git switch 602294-extract-programming-language-resolver

3. Run affected specs:

   bin/rspec spec/lib/gitlab/language_detection_spec.rb spec/lib/gitlab/language_detection/programming_language_resolver_spec.rb spec/services/projects/detect_repository_languages_service_spec.rb

4. Run RuboCop:

   bundle exec rubocop app/services/projects/detect_repository_languages_service.rb lib/gitlab/language_detection.rb lib/gitlab/language_detection/programming_language_resolver.rb spec/lib/gitlab/language_detection_spec.rb spec/lib/gitlab/language_detection/programming_language_resolver_spec.rb spec/services/projects/detect_repository_languages_service_spec.rb

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