fix(core): normalize OAS 3.1 content media schemas

[erichasinternet]

Fixes #23095
Refs #13083, #9083

OpenAPI 3.1 allows a binary string payload to be represented with JSON Schema content keywords, for example:

type: string
contentMediaType: application/octet-stream

OpenAPI Generator already has binary/file handling around the normalized type: string + format: binary shape. This PR normalizes that OAS 3.1 form before schema processing treats type-less content schemas as empty schemas.

With this change, OAS 3.1 contentMediaType: application/octet-stream schemas normalize to type: string and format: binary. It also covers nullable type: [string, "null"] and type-less content schemas. Schemas with contentEncoding, non-binary media types, existing explicit formats, refs, or non-string types are left unchanged.

I added focused coverage for the normalizer and multipart form parameter handling using an OAS 3.1 fixture, and ran the checklist build/sample/docs commands locally.

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