tip: stop empty unknown provider response loop (origin #51290)

[kvnloo]

Origin

Upstream tip (not touched): anomalyco#51290
Closes upstream context: anomalyco#32366

What

Empty provider responses with finish reason unknown must error and idle the session instead of spinning forever.

Proof plan

fail→pass: bun test --timeout 30000 test/cli/run/run-process.test.ts -t "empty unknown" in packages/opencode.

Policy

github_writes_origin=0 — fork only. Codex promote parked.