[FEATURE REQ] Full support for SSE

[XiaofeiCao]

Goal

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

Current state

• Minimal Azure Core support: Add minimal SSE response support to azure-core #50185
• Initial Search implementation: [Search] Add knowledge base retrieve stream convenience APIs #50210
• Closeable stream and event envelope proposal: [azure-core] Add CloseableIterableStream for resource-backed streaming #50081
• SSE design exploration: https://gist.github.com/XiaofeiCao/43c04645e7082a4636990eef5455b95e

Work items

Azure Core contracts

• [FEATURE REQ] Define the synchronous streaming API shape #50414
• [FEATURE REQ] Define the public server-sent event representation #50415

TypeSpec Java emitter

• [http-client-java] Generate HTTP response-to-SSE decoding in service SDKs microsoft/typespec#11941
• [http-client-java] Generate synchronous and asynchronous SSE operations microsoft/typespec#11942
• [http-client-java] Generate polymorphic SSE event models 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