Stream Stall Detection and Recovery

What changed: New error messages and handling for two distinct streaming failure modes:

• "Response stalled mid-stream. The response above may be incomplete." — shown when the stream goes idle after some output has been produced.
• "Response stalled while thinking, before producing a response. Try again." — shown when the stream stalls during the thinking phase before any output.
• "Stream idle timeout after thinking-only yield — retrying streaming" — internal retry logic for the thinking-stall case.

These replace previous silent failures or generic timeout messages with specific, actionable descriptions.