## Why
HTTP 400 responses with `context_length_exceeded` need to reach context-window recovery so manual compaction can retry with less history. Streaming context-window failures should end the stream immediately.
## What changed
- Map `context_length_exceeded` through the shared terminal provider error parser for HTTP 400 responses and events without an HTTP status.
- Return SSE context-window failures immediately.
- Preserve retryable handling for WebSocket errors carrying HTTP 503, even when their code is `context_length_exceeded`.
## Testing
Add coverage for WebSocket status handling and immediate SSE termination with retry headers. Update the manual compaction test to verify recovery from an HTTP 400 context-window error by dropping one history item and retrying.
GitOrigin-RevId: 56d8c831c2fad2ca1318318a3b4258fcc516d689
Charlie Marsh
1885417cd4