[http-client-java] Generate HTTP response-to-SSE decoding in service SDKs

[XiaofeiCao]

Clear and concise description of the problem

The Java emitter needs to convert streaming HTTP responses into SSE events. Per Azure/azure-sdk-for-java#50264, decoding should be generated into each service SDK's implementation package rather than added to Azure Core.

Acceptance criteria

• Generated code handles UTF-8, BOM, SSE line endings, multiline data, comments, and malformed fields.
• Response closure, completion, errors, and cancellation have deterministic behavior.
• Pending line and event data are bounded with clear failures when limits are exceeded.
• Decoder code remains outside the service SDK's public API and has generated tests.

Dependencies

• Parent: [FEATURE REQ] Full support for SSE Azure/azure-sdk-for-java#50264
• Synchronous API contract: [FEATURE REQ] Define the synchronous streaming API shape Azure/azure-sdk-for-java#50414
• Event representation contract: [FEATURE REQ] Define the public server-sent event representation Azure/azure-sdk-for-java#50415

Checklist

• Follow our Code of Conduct
• Read the docs
• Checked for an existing duplicate