feat: adds support for describing SSE in OpenAPI 3.2.0

[baywet]

fixes #64379

Examples

# SseItem<Todo>
responses:
  '200':
    description: OK
    content:
      text/event-stream:
        itemSchema:
          type: object
          required:
            - data
          properties:
            data:
              $ref: '#/components/schemas/Todo'
            event:
              type: string
            id:
              type: string

# SseItem<UnionPet> where UnionPet = Kitten | Puppy
responses:
  '200':
    description: OK
    content:
      text/event-stream:
        itemSchema:
          type: object
          required:
            - data
          properties:
            data:
              $ref: '#/components/schemas/UnionPet'
            event:
              type: string
              enum:
                - Kitten
                - Puppy
            id:
              type: string

# SseItem<Vehicle>, where Vehicle is an abstract base type with JSON discriminators
responses:
  '200':
    description: OK
    content:
      text/event-stream:
        itemSchema:
          type: object
          required:
            - data
          properties:
            data:
              $ref: '#/components/schemas/Vehicle'
            event:
              type: string
              enum:
                - car
                - truck
                - plane
            id:
              type: string

CC @martincostello

[Copilot]

Pull request overview

Adds OpenAPI document generation support for Server-Sent Events (SSE) responses by emitting OpenAPI 3.2-style sequential media type metadata (itemSchema) for text/event-stream, including deriving event enum values from unions and polymorphic discriminator mappings.

Changes:

• Detect text/event-stream responses whose response type is SseItem<T> and emit an SSE “item schema” instead of a normal media-type schema.
• Populate the SSE item schema with data, event, and id properties, and infer event enums from union cases and discriminator mappings when available.
• Add tests validating itemSchema usage and event enum inference for plain, union, and discriminator-based payload types.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/OpenApi/src/Services/OpenApiDocumentService.cs	Detect SSE responses and generate text/event-stream media types using ItemSchema, including event enum inference from unions/discriminators.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiDocumentService/OpenApiDocumentServiceTests.Responses.cs	Adds coverage for SSE itemSchema output and validates event schema behavior for different payload shapes.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.