fix: xAI streaming can deadlock permanently on client disconnect or cancellation

[sam-saffron-jarvis]

What

• make both xAI streaming paths use a context-aware event emitter instead of plain blocking channel sends
• return early when the stream context is cancelled while emitting text, tool execution, usage, or done events
• add regression tests covering both the standard chat stream and the search/responses stream when the consumer stops draining events

Why

If the caller disconnects or abandons the stream, the 16-event buffer created by newEventStream can fill up. The old plain events <- ... sends in internal/llm/xai.go would then block forever, which prevented channelStream.Close() from finishing and left the xAI response body/request wedged. Making those sends context-aware lets cancellation unwind the producer goroutine cleanly.