[pull] main from remix-run:main - #318

…er crash (#15286)

When a client aborts a document request, the readable side of the
injectRSCPayload TransformStream is cancelled while a buffered flush
(setTimeout(..., 0)) may still be pending. The timer then enqueues into
a cancelled stream, throwing "TypeError: Invalid state: Unable to
enqueue" from a timer callback — an unhandled rejection that kills the
server process.

Add a cancel() handler that clears the pending flush, drops the buffer,
and cancels the underlying RSC payload stream, plus cancellation guards
on the flush timer and the RSC writer loop.

Fixes #15275

* add a few more tests

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Co-authored-by: Jacob Ebey <jacob.ebey@live.com>