[9.0.2] Fix infinite spin-wait and missing cancellation propagation in TaskDeduplicator. (https://github.com/bazelbuild/bazel/pull/28938) (#28990)

The test
TaskDeduplicatorTest.executeIfNeeded_executeAndCancelLoop_noErrors
sporadically hung during the ExecutorService.close() call. This was due
to two issues:

1. In executeIfNew, if a thread encountered a RefcountedFuture that was
already canceled (refcount = 0), it would call Thread.yield() and
continue the while(true) loop. It relied on a listener attached to the
canceled future to eventually remove it from the map. However, when
using virtual threads, many threads could enter this spin loop
simultaneously, saturating the underlying carrier threads. This
prevented the listener from being scheduled, creating a deadlock where
the spinning threads never saw the entry removed.

2. RefcountedFuture was not propagating the cancel() call to its
delegate future. This meant that even if all callers canceled their
interest in a task, the task would continue to execute in the
background, wasting resources and increasing contention.

This PR makes the following changes:

1. executeIfNew and maybeJoinExecution are modified to explicitly call
inFlightTasks.remove(key, future) if retain() fails. This ensures the
spin loop is broken immediately by the next thread to encounter the
canceled future, rather than waiting for an asynchronous listener.

2. RefcountedFuture.cancel now calls
delegate.cancel(mayInterruptIfRunning) when the internal reference count
drops to zero.

3. Thread.yield() and the associated
@SuppressWarnings("ThreadPriorityCheck") are removed, as they are no
longer necessary.

Fixes #28302.

Closes #28938.

PiperOrigin-RevId: 883147973
Change-Id: I28c1db252573a4c39b1a9e53d32e218327340054

Commit
https://github.com/bazelbuild/bazel/commit/a0760f1a08aca117912b39b7ba0eda2be50d17ce

Co-authored-by: Tiago Quelhas <tjgq@google.com>