Fix content vs schema

[kain88-de]

Summary

Fix parameter parsing to support OpenAPI parameters defined with content instead of schema.

OpenAPI 3.1 allows a Parameter Object to include either schema or content, but not both:
https://spec.openapis.org/oas/v3.1.0.html#parameter-object

Before this change, content-only parameters were dropped during parsing because only param_schema was considered. That caused:

• path parameters to fail later with an incorrect path templating warning
• query parameters to disappear from generated endpoint signatures

Changes

• fall back to content.<media-type>.schema when schema is missing
• add a parser regression test for content-only parameters
• add a functional happy-path generated-code test
• add 3.1 snapshot coverage for a content-only path parameter