fix(ai-chat): preserve first useAgentChat message during initial URL resolution

[whoiskatrin]

Summary

Fix a mount-time race in useAgentChat where a message sent immediately after first render could disappear from the hook's messages array.

Problem

When useAgentChat mounts before the Agent HTTP URL is available:

1. The hook creates its initial AI SDK Chat instance using a fallback chat ID.
2. The user sends a message immediately, so the message is optimistically added locally.
3. The Agent HTTP URL becomes available on a later render.
4. useAgentChat upgrades the chat ID to a URL-derived ID.
5. The AI SDK recreates the underlying Chat instance, which drops the optimistic message state.

Depending on timing, the first sent message would not remain visible in messages.

Fix

Keep the initial Chat ID stable across the HTTP URL transition for the same mounted Agent instance.

The hook still recreates the chat when the consumer genuinely switches to a different Agent/chat, but it no longer recreates the chat just because the Agent URL becomes available after mount.

When initial persisted history resolves after an optimistic first send, late hydration now prepends missing hydrated messages instead of overwriting or discarding either side. Existing in-memory messages win by ID so newer local/stream state is not replaced by the fetch response.

Regression tests

Added React regression coverage for:

• mounting useAgentChat before getHttpUrl() is ready
• immediately calling sendMessage()
• making the HTTP URL become available
• verifying the optimistic first message remains present
• verifying late persisted history and the optimistic first send are both retained

Validation

Passed:

• npm run test:react --workspace packages/ai-chat
• npx oxfmt --check packages/ai-chat/src/react.tsx packages/ai-chat/src/react-tests/use-agent-chat.test.tsx

Also ran npm run check. Sherif, export validation, repository formatting, and Oxlint completed successfully; the typecheck stage could not execute in this local environment because tsgo was unavailable (/bin/sh: tsgo: command not found).

Release note

Includes a patch changeset for @cloudflare/ai-chat.

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🦋 Changeset detected

Latest commit: 8677a96

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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commit: 8677a96

[devin-ai-integration]

✅ Devin Review: No Issues Found

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