fix(ai-chat): abort/cancel support for streaming responses

[threepointone]

Summary

Fixes the stop button in AI Chat so it properly aborts streaming responses, and adds a framework-level safety net for cancellation.

What changed

Framework (@cloudflare/ai-chat)

• Abort signal propagation in _reply, _streamSSEReply, _sendPlaintextReply: When the abort signal fires, the reader loop is now cancelled and a done signal is sent to the client. Previously, cancelling a request would send the cancel message but the server-side stream would keep running until it finished naturally.
• Warning log: If cancellation arrives but the stream has not closed, a warning is logged suggesting the developer forgot to pass abortSignal to their LLM call (e.g. streamText). This helps catch a common misconfiguration.

Example (examples/ai-chat)

• Pass options.abortSignal through to streamText in server.ts so the upstream LLM call is actually cancelled when the user hits stop.

Test infrastructure

• Added cancel-request.test.ts with 5 tests covering: plaintext cancel, SSE cancel, abort controller cleanup, cancel with abortSignal wired, and full stream completion without cancel.
• Added SlowStreamAgent test fixture that streams chunks with configurable delays.
• Fixed vitest project configs (src/tests/vitest.config.ts, src/react-tests/vitest.config.ts) to add explicit include patterns. Without these, vitest's default glob was picking up e2e/*.spec.ts files and trying to run them in the Workers pool (causing No such module "node:child_process" errors) and react-test files in the wrong runner.

Reviewer notes

• The core abort logic lives in packages/ai-chat/src/index.ts around the _reply, _streamSSEReply, and _sendPlaintextReply methods. Look for the abortSignal event listener pattern — when abort fires, we cancel the reader, send the done signal, and log a warning if the stream was still open.
• The vitest config fix is minimal (include globs) but important — without it, npm run test from the ai-chat package root would fail on CI because Playwright e2e tests and browser-mode react tests got picked up by the Workers pool. The e2e tests are intentionally separate and run via npm run test:e2e / the nightly CI workflow.
• The SlowStreamAgent in worker.ts is a test-only Durable Object. It simulates a slow LLM by yielding chunks with setTimeout delays, which gives the cancel tests enough time to send a cancel message mid-stream.
• All 223 existing tests continue to pass, plus the 5 new cancel tests.

Testing

# Unit/integration tests (workers pool)
cd packages/ai-chat && npx vitest run

# 25 files, 223 tests — all pass

[changeset-bot]

🦋 Changeset detected

Latest commit: 0ac1fa2

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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