Replace UAF death test with deterministic stale-delegate assertion

[fkgozali]

Summary:
SchedulerDelegateInvalidationTest.DelegateDestroyedWithoutError_PendingRenderingUpdateIsUAF
asserted a real use-after-free through EXPECT_DEATH: it destroyed the
RecordingDelegate, then drained pendingRenderingUpdates_ so the queued lambda
dereferenced the freed object, and expected the process to die. Undefined behaviour
is not a reliable process-termination signal, without a sanitizer the freed read
can simply succeed, and gtest then reports Result: failed to die. The death test
also has to fork() a multi-threaded process.

This replaces the death test with a deterministic assertion on the same property.
Instead of destroying the delegate, the test detaches it via
Scheduler::setDelegate(nullptr) and keeps it alive, then drains the pending
rendering update and asserts that the drained lambda still invokes
schedulerShouldRenderTransactions on the detached delegate.

That pins exactly the coverage the death test was after: Scheduler::setDelegate is
a plain assignment, so a lambda already queued by uiManagerDidFinishTransaction
keeps the raw delegate pointer it captured, and draining it after the delegate has
been detached still calls through that pointer, which is a use-after-free when the
delegate has been destroyed rather than merely detached. Same property, no undefined
behaviour and no fork(). It matches the shape of the existing
UnregisterSurface_DoesNotDrainPendingRenderingUpdates test in the same file.

The underlying window is unchanged: nothing in Scheduler::setDelegate cancels
rendering updates that are already queued, so closing it needs a shutdown signal at
the runtime-scheduler level. That is a design decision for the owners rather than a
test fix.

Changelog: [Internal]

Differential Revision: D118205636

[meta-codesync]

@fkgozali has exported this pull request. If you are a Meta employee, you can view the originating Diff in D118205636.

[cortinico]

Review automatically exported from Phabricator review in Meta.

[meta-codesync]

This pull request has been merged in d9ad3f0.