test(llm): verify timeout cancels response stream

[LeXwDeX]

Summary

• exercise the public LLM stream through the production fetch-backed HTTP transport
• use a real loopback Bun server with bounded request-received and response-canceled fences
• assert the existing LLMError Transport/Timeout shape and server-observed response cancellation

Verification

• target suite passed three consecutive runs: 7 pass, 0 fail each
• bun typecheck passed in packages/llm
• mutation check: removing response stream timeout made the new scenario time out and fail; mutation restored

No production code changes were required.