chore(sdk): drop stale seedSessionInTail comment

The comment described metadata: undefined; seeded records now carry the
harness clientData. The code is self-explanatory, so remove the comment
rather than reword it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VG39FXXkFFU24U5EtJMwPi

Author: ericallam

packages/trigger-sdk/src/v3/test/mock-chat-agent.ts

@@ -488,11 +488,6 @@ export function mockChatAgent(
     } as never;
   });
 
-  // session.in tail override: each seeded UIMessage becomes a
-  // { message, metadata: undefined, seqNum: i+1 } entry. Mirrors the
-  // seq-num pattern from the out-tail stub so cursor-advance logic is
-  // exercised correctly. `metadata` is `undefined` for seeded users —
-  // the boot path falls back to `payload.metadata` for those.
   __setReplaySessionInTailImplForTests(async () => {
     return seededSessionInMessages.map((message, i) => ({
       message,