[http-client-java] Generate synchronous and asynchronous SSE operations

### Clear and concise description of the problem

The Java emitter does not generate complete public APIs and wiring for TypeSpec SSE operations.

### Acceptance criteria

- Generates the approved synchronous consumption type and asynchronous `Flux` type.
- Connects operation methods to generated SSE decoding.
- Generates required imports, annotations, JavaDoc, and tests.
- Covers normal completion, early termination, errors, and cancellation.

### Dependencies

- Parent: Azure/azure-sdk-for-java#50264
- Synchronous API contract: Azure/azure-sdk-for-java#50414
- Generated decoder: microsoft/typespec#11941

### 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