fix(ai-chat): terminate WebSocket chat transport stream on abort

[threepointone]

Summary

Fixes #985

The ReadableStream returned by WebSocketChatTransport.sendMessages() was never terminated when the caller's AbortSignal fired or stream.cancel() was called. This left useChat stuck in the "streaming" state after the user clicked Stop/Cancel.

What changed

packages/ai-chat/src/ws-chat-transport.ts

• All three terminal paths (done, error, abort) now funnel through a single finish(action) helper that gates on a completed flag, cleans up activeRequestIds, and detaches the WS listener via abortController.abort().
• onAbort() sends CF_AGENT_CHAT_REQUEST_CANCEL to the server (best-effort), then errors the stream with an AbortError via finish().
• stream.cancel() delegates to onAbort(), so cancelling the stream from the consumer side also notifies the server.
• The caller's abortSignal listener is registered with { once: true } and handles already-aborted signals.

packages/ai-chat/src/tests/ws-transport-abort.test.ts (new)

Four test cases exercising the transport directly:

1. Mid-stream abort — aborts after 200ms, verifies the stream terminates (the original regression)
2. Pre-aborted signal — passes an already-aborted signal, verifies immediate termination
3. stream.cancel() — verifies cancel completes without hanging
4. Normal completion — sanity check that the happy path still works through the transport

How this differs from the proposed fix in #985

The community fix is correct and solves the bug, but our implementation simplifies the approach:

Aspect	Proposed fix	This PR
State flags	Two flags (abortRequested + completed)	One flag (completed)
Cleanup helpers	Two functions (requestAbort + completeAbort)	One function (finish)
start() race guard	Checks abortRequested in start() to handle "abort before stream starts"	Removed — start() runs synchronously during ReadableStream construction, so streamController is always set before any listener can fire
Cleanup duplication	done/error paths in onMessage have separate inline cleanup	All paths use finish()
Tests	1 test (mid-stream abort)	4 tests

Notes for reviewers

• Definite assignment assertion: let streamController!: ReadableStreamDefaultController uses ! which is new to this codebase. The comment above it explains why it is safe (synchronous start()). If you prefer | undefined with optional chaining, happy to change.
• Pre-aborted signal still sends the WS request: If the signal is already aborted when sendMessages() is called, onAbort() fires immediately but the request is still sent on the next line. This is harmless — the server receives the cancel and stops, and the client stream is already errored. Adding a guard would be possible but adds complexity for a negligible edge case.
• Hibernation: This change is client-side only and does not affect the server-side resumable streaming mechanism. Stream resumption after hibernation/reconnect flows through the useAgentChat hook's onAgentMessage handler (CF_AGENT_STREAM_RESUMING protocol), which is a separate path from the transport's ReadableStream.
• All 206 tests pass across 25 test files (npx vitest --project workers --run).

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Latest commit: a3c2897

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

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Name	Type
@cloudflare/ai-chat	Patch

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