The deferred write introduced by commit 65aa8f6 ("stream: fix
pipeTo to defer writes per WHATWG spec") wraps the write operation in
queueMicrotask(). A race condition exists where the pipe can shutdown
and release the writer between when [kChunk] schedules the microtask
and when it executes, causing an ERR_INTERNAL_ASSERTION because
writer[[stream]] is undefined.

Fix by moving the shuttingDown flag into the shared pipe state object
so that PipeToReadableStreamReadRequest can check it. The microtask
now skips the write if the pipe has already begun shutting down.

This aligns with the WHATWG Streams spec step 15 which states:
"if shuttingDown becomes true, the user agent must not initiate
further reads from reader, and must only perform writes of
already-read chunks".

Fixes: nodejs#63732
Signed-off-by: Qingyu Wang <wangqingyu.c0l1n@bytedance.com>

The state.shuttingDown guard introduced in the previous commit is too
broad: it skips writes even when the destination is still writable.
The WHATWG Streams spec requires that already-read chunks are written
to a still-writable destination during shutdown (step 15, shutdown
substeps 3.1-3.2).

Replace the guard with a check for writer[kState].stream === undefined
which is only true after finalize() has released the writer. This
precisely targets the crash condition without violating the spec
requirement.

Restore shuttingDown as a closure-local variable since it no longer
needs to be shared with PipeToReadableStreamReadRequest.

Add a test case verifying that an already-read chunk is written when
an AbortSignal fires after enqueue while the destination is still
writable.

Signed-off-by: Qingyu Wang <wangqingyu.c0l1n@bytedance.com>