[http-client-java] Generate polymorphic SSE event models

### Clear and concise description of the problem

The Java emitter needs a consistent model shape for SSE operations that can emit multiple service-defined event kinds.

### Acceptance criteria

- Selects and generates one representation, such as per-event wrappers, a marker hierarchy, or a service union.
- Preserves discriminator information for known event kinds.
- Defines behavior for unknown and terminal event kinds.
- Maintains forward-compatible deserialization and includes generated tests.

### Dependencies

- Parent: Azure/azure-sdk-for-java#50264
- Public event representation contract: Azure/azure-sdk-for-java#50415

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/)
- [x] Checked for an existing duplicate