Add explicit discriminator mappings for Realtime session configuration unions

The Realtime session configuration union is currently represented using oneOf in:

• RealtimeClientEventSessionUpdate.properties.session
• RealtimeCreateClientSecretRequest.properties.session

Both referenced schemas require a type property with distinct constant values:

• RealtimeSessionCreateRequestGA: type: realtime
• RealtimeTranscriptionSessionCreateRequestGA: type: transcription

However, the union does not explicitly declare its discriminator. This prevents some OpenAPI consumers and code generators from recognizing it as a discriminated union.

Could the schema include an explicit discriminator mapping?

session:
  type: object
  description: |
    Update the Realtime session. Choose either a realtime
    session or a transcription session.
  oneOf:
    - $ref: "#/components/schemas/RealtimeSessionCreateRequestGA"
    - $ref: "#/components/schemas/RealtimeTranscriptionSessionCreateRequestGA"
  discriminator:
    propertyName: type
    mapping:
      realtime: "#/components/schemas/RealtimeSessionCreateRequestGA"
      transcription: "#/components/schemas/RealtimeTranscriptionSessionCreateRequestGA"

Alternatively, the union could be extracted into a reusable component and referenced from both properties.

This would preserve the existing wire format while making the intended polymorphism explicit for OpenAPI tooling.

Locate the schemas for RealtimeClientEventSessionUpdate.properties.session and RealtimeCreateClientSecretRequest.properties.session, then read the referenced RealtimeSessionCreateRequestGA and RealtimeTranscriptionSessionCreateRequestGA definitions. Add the explicit type discriminator mapping to both unions, or identify a reusable component if the repository structure supports it. Done means the existing wire format is unchanged and both session variants are recognized as a discriminated union.