fix: Copilot provider can hang forever when a stream consumer goes away

[sam-saffron-jarvis]

What

• make Copilot chat-completions streaming emit events through a context-aware helper instead of blocking channel sends
• stop the producer loop cleanly when the stream context is cancelled while emitting text, tool-call, usage, or done events
• add a regression test covering stream closure when the consumer stops reading mid-stream

Why

Copilot was writing directly to the internal event channel. If a consumer disconnected or cancelled and stopped draining events, the buffered channel could fill and leave the producer goroutine blocked forever on events <- .... That prevented Close() from finishing, leaked the HTTP response body, and could leave runs or sessions stuck instead of shutting down cleanly.