[http-client-python] Add SSE reconnection

[l0lawrence]

Structured SSE streams generated by @typespec/http-client-python parse and expose stream.last_event_id and stream.retry, and operations accept last_event_id= to send Last-Event-ID for manual resume. They intentionally do not automatically reconnect after an unexpected EOF.

Add automatic SSE reconnection once the behavior is available in the azure-core streaming/transport layer rather than implementing a generator-vendored reconnect loop. The integration should:

• Reconnect after unexpected EOF using the SSE default delay, overridden by valid retry: fields.
• Send the latest event ID through Last-Event-ID.
• Stop reconnecting after terminal events and HTTP 204 responses.
• Preserve normal 301/307 redirect handling through the pipeline.
• Support sync and async streams.
• Fix the Streaming_Sse_Protocol_reconnect Spector scenario.

Related Spector coverage issue: #11572.

[github-actions]

🎯 Issue Triage

Feature request to add automatic SSE reconnection in @typespec/http-client-python, delegating reconnect logic to the azure-core streaming/transport layer rather than implementing it in the generator.

Category: Feature Request
Area: emitter:client:python

🔍 Analysis

• The Python client currently exposes stream.last_event_id and stream.retry and accepts last_event_id= for manual resume, but does not auto-reconnect on unexpected EOF.
• This feature is intentionally deferred until azure-core provides the reconnect primitive, keeping generated code clean and avoiding a generator-vendored reconnect loop.
• Requirements include: honoring retry: delay fields, sending Last-Event-ID, stopping on terminal events or HTTP 204, preserving redirect handling, supporting sync and async, and covering the Streaming_Sse_Protocol_reconnect Spector scenario.
• Related Spector coverage issue: [SSE] Add Spector coverage for id, retry, and reconnection #11572.

📝 Suggested next steps

• Track azure-core upstream for availability of the streaming/transport reconnect primitive.
• Implement integration in @typespec/http-client-python once azure-core support lands.
• Add test coverage for the Streaming_Sse_Protocol_reconnect Spector scenario ([SSE] Add Spector coverage for id, retry, and reconnection #11572).

Generated by Agentic Triage

[l0lawrence]

The removed generator-owned reconnect implementation and its sync/async tests are preserved as a re-applicable patch here: https://gist.github.com/l0lawrence/cb56b0b006cae9f486db9a8a5e8ef85d

This captures the proof-of-concept reconnect loop, generated callback, retry delay, and Last-Event-ID flow