Fix React act environment setup in vitest browser tests

[threepointone]

sunil: Since we were using plain react act() calls, I had to add a setup file for vitest that enabled the IS_REACT_ACT_ENVIRONMENT flag. Then I discovered that vitest-browser-react@1 has a bug where it would set the global IS_REACT_ACT_ENVIRONMENT to false after a run of it's own render (which wraps act()), so if you ran react's own act() after it, you'd get the "The current testing environment is not configured to support act(...)" error. After fixing that, react correctly identified some tests had mutations that weren't wrapped with act, so I fixed them with a small sleep helper.

copilot: Adds a setup file to properly set the IS_REACT_ACT_ENVIRONMENT global for React tests, updates vitest config to use this setup, and patches vitest-browser-react to restore the environment variable after act calls. Also refactors tests to use async act and a sleep helper for more reliable async behavior.

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[claude]

Claude Code Review

Issues:

1. Hardcoded sleep timing (packages/ai-chat/src/react-tests/use-agent-chat.test.tsx:13-15): The sleep(10) calls use an arbitrary 10ms delay. This creates brittle tests that could fail on slower CI runners or pass incorrectly on fast machines. Consider using proper async patterns like waitFor from testing-library or event-based synchronization instead of arbitrary timeouts.

2. Inconsistent async test pattern (multiple locations): Some tests mix await sleep(10) after render() while one test (around line 487-507) had a more complex pattern with an additional 100ms wait that was removed. The removal of that wait suggests timing dependencies that should be addressed with proper synchronization primitives rather than sleeps.

3. Patch maintenance burden (patches/vitest-browser-react+1.0.1.patch): The patch fixes a legitimate bug in vitest-browser-react, but patching node_modules creates long-term maintenance issues:

   • Requires re-applying on every package update
   • May break silently if the library's internal structure changes
   • Consider opening an upstream PR to vitest-browser-react to fix this properly

4. Missing changeset: The PR doesn't include a changeset file for @cloudflare/ai-chat to document this test infrastructure improvement for changelog generation.

Minor:

• The patch file comment at line 9 has a trailing space

The core fix (preserving IS_REACT_ACT_ENVIRONMENT) is correct and addresses the immediate issue, but the sleep-based synchronization approach is fragile.

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commit: 0b4e75c

[anonrig]

ps: node:timers/promises will actieve the same thing

await setTimeout(number)

[threepointone]

nerrrrd