Implement cancellation in ReadableStream handling

[wardpeet]

Add cancellation support to the ReadableStream in the HTTP server.

Related issue mastra-ai/mastra#13899

[roaminro]

I tested this patch against a deterministic MCP Streamable-HTTP repro and found one additional cancellation-propagation requirement. The proposed cancelled guard prevents the fatal ERR_INVALID_STATE, but cancel() only sets a local flag; it does not notify consumers of the simulated Node ServerResponse.

In this path, @hono/node-server is piping an inner Web ReadableStream into FetchServerResponse and listens for writable.on("close", () => reader.cancel()). Without a close event, its reader is never cancelled, so @modelcontextprotocol/sdk never runs its inner ReadableStream.cancel() and its 15-second SSE keepalive interval remains orphaned, writing forever into the now-discarded response.

Adding close propagation in addition to this PR's guards fixed both the crash and the timer leak:

cancel() {
  cancelled = true;
  _this.emit("close");
}

(or an equivalent destroy/close implementation that emits close). One already-in-flight buffered write can land in the same millisecond as cancellation, so the existing cancelled guard/try-catch is still necessary.

Proof: one MCP client connect+disconnect kills the unpatched server exactly at the SDK's 15,000ms keepalive. With the guards + close propagation, instrumentation shows outer cancel -> inner MCP cancel, no writes at 15s/30s, and Mastra's full API suite passes 363/363. Full trace: mastra-ai/mastra#20332 (comment)