fix(backend): register AutoPilot sessions with stream registry for SSE updates (#12500)

### Changes 🏗️
- When the AutoPilot block executes a copilot session via
`collect_copilot_response`, it calls `stream_chat_completion_sdk`
directly, bypassing the copilot executor and stream registry. This means
the frontend sees no `active_stream` on the session and cannot connect
via SSE — users see a frozen chat with no updates until the turn fully
completes.
- Fix: register a `stream_registry` session in
`collect_copilot_response` and publish each chunk to Redis as events are
consumed. This allows the frontend to detect `active_stream=true` and
connect via the SSE reconnect endpoint for live streaming updates during
AutoPilot execution.
- Error handling is graceful — if stream registry fails, AutoPilot still
works normally, just without real-time frontend updates.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] Trigger an AutoPilot block execution that creates a new chat
session
- [x] Verify the new session appears in the sidebar with streaming
indicator
- [x] Click on the session while AutoPilot is still executing — verify
SSE connects and messages stream in real-time
- [x] Verify that after AutoPilot completes, the session shows as
complete (no active_stream)
- [x] Test reconnection: disconnect and reconnect while AutoPilot is
running — verify stream resumes (found and fixed GeneratorExit bug that
caused stuck sessions)
- [x] E2E: 10 stream events published to Redis (StreamStart,
3×ToolInput, 3×ToolOutput, TextStart, TextEnd, StreamFinish)
  - [x] E2E: Redis xadd latency 0.2–3.4ms per chunk
  - [x] E2E: Chat sessions registered in Redis (confirmed via redis-cli)