fix(ai-chat): preserve server-generated messages when client appends new messages

[threepointone]

Problem

The _deleteStaleRows reconciliation in persistMessages (added in #1014 for regenerate support) was too aggressive. When a client sent a chat request with new messages that didn't include the full server history, it deleted server-generated assistant messages the client hadn't seen yet.

This caused the "multiple messages accumulate in conversation" e2e test to fail: the second request's persistMessages call deleted the first assistant message because it wasn't in the incoming message set.

Fix

Gate stale row deletion on whether the incoming messages are a subset of the server state. This distinguishes:

• Regenerate/trim: all incoming IDs exist in server state → delete stale rows ✓
• Append new messages: incoming set contains IDs unknown to server → skip deletion, preserve server messages ✓

Changes

• packages/ai-chat/src/index.ts: Add isSubsetOfServer guard before stale row deletion
• packages/ai-chat/src/tests/regenerate-message.test.ts: Add regression test

Testing

• 33/33 e2e tests pass (including previously failing "multiple messages accumulate")
• 291/291 unit tests pass (including all regenerate tests + new regression test)

[changeset-bot]

🦋 Changeset detected

Latest commit: 03fe9c9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@cloudflare/ai-chat	Patch

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