Parse media types with mime

[danieleades]

Summary

• declare mime as a direct dependency of progenitor-impl
• parse request body content types with mime::Mime and classify them by their parameter-free essence
• use the same MIME parsing behavior when detecting JSON responses
• add focused coverage for parameters, case normalization, unsupported structured suffixes, and malformed media types

Why

Media types were previously handled with two separate string-based approaches: request bodies sliced at the first semicolon, while responses used equality and prefix checks. Using a media type parser centralizes the syntax handling and makes the supported content-type policy explicit.

Impact

Existing supported body types retain their generated behavior. Valid media type parameters and case variations are handled consistently, while application/problem+json remains unsupported rather than implicitly changing the generator's structured-suffix policy.

Validation

• cargo fmt --all -- --check
• cargo test -p progenitor-impl (17 unit tests, 9 output tests, and 3 non-network specific tests passed in the sandbox)
• cargo test -p progenitor-impl --test test_specific (4/4 passed with localhost binding enabled)