fix(api): document ratings response variants

[kevincador]

Summary

Correct the ratings endpoint contracts and documentation to represent both
response forms:

• The default response returns rating, votes, and distribution at the top
  level.
• extended=all returns a nested trakt object plus supported external rating
  providers.

Closes #872

Changes

• Constrain the ratings extended query parameter to all.
• Model both existing response shapes using codegen-safe optional fields.
• Add named default and extendedAll OpenAPI response examples.
• Document supported providers for movies, shows, seasons, and episodes.
• Mark extended-only provider blocks, including metascore, as optional.

Risk

Low. The response type is widened to represent both payloads already returned
by the API. Generated clients cannot statically select a response shape from a
query parameter, so the two variants are distinguished through named examples
and endpoint documentation.