feat: support OpenAPI 3.2 itemSchema (SSE/streaming)

[dkachur1]

This adds support for the itemSchema field introduced in OpenAPI 3.2, which defines the shape of individual items in streaming responses like text/event-stream.

What changed

• MediaTypeObject now recognizes itemSchema. When present, it takes precedence over schema for type generation.
• Upgraded @redocly/openapi-core to v2 to support OAS 3.2 parsing.

Example

OpenAPI 3.2

  "200":
    content:
      text/event-stream:
        itemSchema:
          oneOf:
            - type: object
              properties:
                type: { type: string, enum: [content] }
                text: { type: string }
              required: [type, text]
            - type: object
              properties:
                type: { type: string, enum: [done] }
              required: [type]

Generates:

  "text/event-stream": {
      type: "content";
      text: string;
  } | {
      type: "done";
  };

What's supported

• itemSchema with objects, primitives, arrays, enums, oneOf, allOf
• itemSchema with $ref to component schemas
• Mixed responses (application/json with schema + text/event-stream with itemSchema)
• Backward compatible — specs without itemSchema work exactly as before

Test plan

• Unit tests for transformMediaTypeObject (precedence, fallback, backward compat)
• Integration tests through response and request body transforms
• E2E test with $ref resolution in itemSchema
• E2E test with a full OpenAPI 3.2 YAML fixture covering SSE endpoints with oneOf, enums, and POST request bodies

---

This is my first open source contribution — I'm a student, so any feedback is very welcome!

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The changes in this PR will be included in the next version bump.

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openapi-typescript	Minor

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