EventQueueProcessor/EventTarget: SCOPE_EXIT-guard release on throwing dispatch + fix unsigned retainCount_ underflow (#56490)

[meta-yaohway]

Summary:
1) EventQueueProcessor::flushEvents — release on every exit path. flushEvents retains every EventTarget in the batch, then for each event calls eventPipe_ (which dispatches into JS via UIManagerBinding::dispatchEventToJS and can throw jsi::JSError), then eventPipeConclusion_, and finally runs a trailing loop that calls event.eventTarget->release(runtime). If either pipe call throws, stack unwinding skips the release loop, leaving strongInstanceHandle_ (a jsi::Value strong reference to the JS event-target object) pinned and retainCount_ desynchronised.

This change replaces the trailing release loop with a folly SCOPE_EXIT block immediately after the retain pass, so every retained target is released on every exit path — normal return, eventPipe_ throw, or eventPipeConclusion_ throw. (Earlier revisions used a hand-rolled RAII class; switched to SCOPE_EXIT per reviewer feedback. folly/ScopeGuard.h is already in the OSS RN folly subset — RCT-Folly.podspec, ReactAndroid/.../folly/CMakeLists.txt, react-native-fantom/.../folly/CMakeLists.txt — and is a transitive dep of //xplat/folly:dynamic, so no new build edges.)

Behaviour-preserving on the happy path: same release ordering (after eventPipeConclusion_), same no-DispatchMutex policy for release (the events vector still holds the strong shared_ptrs), zero added allocation.

2) EventTarget::release — guard the unsigned decrement. retainCount_ is size_t. retain() early-returns when the target is disabled, but the matching release() always runs, so release() can be entered with retainCount_ == 0; the unconditional --retainCount_ then wraps to SIZE_MAX, the == 0 check never fires again, and strongInstanceHandle_ is never cleared. The trailing react_native_assert(retainCount_ >= 0) is a tautology on an unsigned type and is removed. This change guards the decrement (if (retainCount_ > 0) --retainCount_;) and drops the dead assert and the now-unused react_native_assert.h include.

Adds EventQueueProcessorTest.releasesEventTargetsWhenDispatchThrows (proves the strong handle is released during unwind when eventPipe_ throws; fails on the pre-fix trailing-loop code) and EventTargetTests.releaseWhileDisabledDoesNotCorruptRetainCount (retain/release while disabled, then enable+retain must still acquire the handle; fails on the pre-fix unconditional decrement).

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D100280132

[meta-codesync]

@meta-yaohway has exported this pull request. If you are a Meta employee, you can view the originating Diff in D100280132.

[react-native-bot]

This pull request was successfully merged by @meta-yaohway in ccff70b

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[meta-codesync]

This pull request has been merged in ccff70b.