Migrate Library Contents API to FastAPI

[arash77]

What did you do?

• Include an API test under lib/galaxy_test/api/
• Refactor the legacy library_contents API logic so all the logic is contained in the LibraryContentsService class
• Create pydantic models for the responses and payloads
• Create the FastAPI routes that will replace the old ones

related to #10889

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.