Fix Response Schema Handling with Custom MIME Types

[fosple]

This PR addresses an issue where response schemas weren't properly associated with custom MIME types. In the original code it was marked with a TODO. When using @produce with a custom MIME type (like json-api), schema references weren't being correctly linked to those MIME types in the OpenAPI spec output.

Changes:

• Modified ResponseSchemaWithCustomMimeTypeV3 implementation to ensure schemas are correctly linked to the specified MIME types in the generated spec
• Added comprehensive tests to verify correct behavior for:
  • Custom MIME types (json-api)
  • Default fallback to application/json when no MIME type specified
  • Multiple MIME types sharing the same schema reference

The changes ensure that when you specify @Produce json-api followed by a response schema via @Success, the schema is correctly linked to the application/vnd.api+json MIME type in the OpenAPI document, rather than being linked to the default application/json type.

This fixes issues when generated clients need to consume non-standard response content types.