Address non-blocking review items on the AI-chat stream-perf PR:
- Drop the unused `metadata` param from the `msg` test factory in
message-item.test.ts; no caller passed it.
- Add a per-part-kind coupling guard to message-signature.test.ts that, for
each part kind rendered today (text, reasoning, tool-*) plus the metadata
banners, asserts that mutating a field the MessageItem render body DRAWS
flips messageSignature — an executable lock for the load-bearing memo
invariant documented in message-signature.ts.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
cb61274187