[FEATURE REQ] Full support for SSE

# Goal

Provide consistent synchronous, asynchronous, and generated SSE support for Azure Java client libraries.

## Current state

- Minimal Azure Core support: #50185
- Initial Search implementation: #50210
- Closeable stream and event envelope proposal: #50081
- SSE design exploration: https://gist.github.com/XiaofeiCao/43c04645e7082a4636990eef5455b95e

## Work items

### Azure Core contracts

- [ ] #50414
- [ ] #50415

### TypeSpec Java emitter

- [ ] microsoft/typespec#11941
- [ ] microsoft/typespec#11942
- [ ] microsoft/typespec#11943

## Dependency order

1. Define the synchronous API shape and public event contracts.
2. Generate response-to-SSE decoding in each service SDK's implementation package.
3. Generate public sync/async operations and polymorphic event models.

## Current decisions

- Keep response-to-SSE decoding in generated service SDK implementation code rather than Azure Core.
- Use Search as the initial validation target.

## Deferred

- Automatic reconnect
- `Last-Event-ID` resumption
- Retry policy and recovery guidance