[Fizz] Finish abort in a scheduled task (#36580)

Stacked on #36584

`abort()` currently performs both the synchronous transition into an
aborted request and the reporting/completion of every unfinished task in
the same call. This change splits those phases. Aborting now
synchronously marks the request as aborted, captures the abort reason,
claims pending tasks so already scheduled work cannot continue rendering
them, and captures any DEV async debug information needed at the point
of abort. Reporting and completing the claimed tasks is then performed
from a scheduled `finishAbort()` callback.

This split does not yet allow a promise rejected by an abort listener to
replace the abort reason: work remains blocked once the request has been
aborted, and tests assert that abort-time rejections still report the
original abort reason. It establishes the task boundary needed for a
follow-up change to selectively process rejected suspended work before
completing the remaining aborted tasks.

This is observable for streaming renders because abort cleanup may now
happen after already available output is read. A Suspense boundary that
was previously converted to client rendering before it could be
serialized may instead be emitted as pending first and receive its
client-render instruction when the scheduled abort completion runs.

The scheduled finish must also preserve abort-during-render behavior in
renderers whose scheduler executes synchronously. The request tracks its
currently executing task, and both abort phases leave that task alone so
it can unwind through its normal abort path rather than being completed
twice or reporting an internal control-flow value.