Replace UAF death test with deterministic stale-delegate assertion - #58321

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