Support RFC 6839 +json suffix for vendor MIME types

[szhajdu]

Problem

The generator only supports a hardcoded list of content types (application/json, application/vnd.api+json, application/xml, application/x-www-form-urlencoded). Many APIs return vendor-specific JSON MIME types that follow the RFC 6839 structured syntax suffix convention, such as:

• application/vnd.sdmx.data+json (European Central Bank API)
• application/hal+json (HAL)
• application/ld+json (JSON-LD)
• application/problem+json (RFC 7807)

Currently, using these content types in an OpenAPI spec results in:

Error on mapping operationName: Response content-type ["application/vnd.sdmx.data+json"] is not currently supported.

Reproduction

responses:
  "200":
    description: Success
    content:
      application/vnd.sdmx.data+json:
        schema:
          $ref: "#/components/schemas/MySchema"

Proposed Solution

Per https://datatracker.ietf.org/doc/html/rfc6839, MIME types with a +json suffix indicate JSON-based formats. The generator should:

1. Accept +json suffixed types during generation (src/Entity/Request.php, src/Entity/Response.php)
2. Fall back to JSON serializer at runtime for any +json type (src/Output/Copy/Serializer/BodySerializer.php)
3. Auto-register JSON serializer when any +json type is detected (src/Generator/ServiceProviderGenerator.php)

This approach is:

• RFC compliant
• Backwards compatible
• Requires no configuration
• Automatically supports all vendor JSON formats

I'm happy to submit a PR for this if the approach sounds reasonable.