Fix flaky React and chat concurrency tests

[threepointone]

Summary

• Reuse an already-started React test worker during browser global setup instead of killing a healthy worker on the shared test port.
• Make the useAgent server-broadcast state test use two clients in a unique room so it exercises the actual server broadcast path.
• Give debounce/merge concurrency tests enough first-turn runtime for overlapping submits to be admitted before supersession checks run.

Test plan

• npx vitest --run -c packages/agents/src/react-tests/vitest.config.ts packages/agents/src/react-tests/useAgent.test.tsx
• npx vitest --run -c packages/ai-chat/src/tests/vitest.config.ts packages/ai-chat/src/tests/message-concurrency.test.ts
• npm run check

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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commit: 82815ba