Fix "Cannot read property 'canonical' of undefined" in Fabric getListener when event target is not a live fiber - #58418

Summary:

Fixes a production crash in the Fabric legacy event system:

 TypeError: Cannot read property 'canonical' of undefined
 at getListener (ReactFabric-dev.js:997)

getListener guards the fiber's stateNode with a strict null check:

inst = inst.stateNode;
if (null === inst) return null;
inst = getFiberCurrentPropsFromNode(inst); // → instance.canonical.currentProps

Inside the reconciler this is sufficient — stateNode is initialized to null in the
FiberNode constructor, reset to null in detachFiberAfterEffects (including the
alternate), and only ever assigned instance objects in completeWork — so undefined
cannot be produced by React itself.

However, the responder event path receives its target from native dispatch across the
JSI boundary. The instance handle is stored natively as a weak reference
(EventTarget/InstanceHandle, where a dead jsi::WeakObject::lock() yields
undefined rather than null), so when a touch is delivered for a view that unmounted
mid-gesture (screen transition on press, sheet dismissal, list cell recycling), the
target reaching the plugins is no longer guaranteed to be a live host fiber. Reading
.stateNode on such a target yields undefined, which passes the strict === null
check and crashes one line later in getFiberCurrentPropsFromNode with the error above.

This change loosens the check to null == inst, matching the guards already used at the
other entry points of the same pipeline (null != target in dispatchEvent,
null == targetInst in ReactNativeBridgeEventPlugin.extractEvents). With the guard,
the event for a dead target is dropped instead of throwing.

We are seeing this crash in production (React Native 0.84.0, Hermes, new architecture)
reported to Sentry as an unhandled error, correlated with touches during unmount.

Note: ReactFabric-{dev,prod,profiling}.js are generated bundles synced from
facebook/react — the source counterpart of this fix is
packages/react-native-renderer/src/ReactNativeGetListener.js (and its inlined copy in
ResponderEventPlugin). Happy to redirect this patch there if that is the preferred
route; filing here first since the crash manifests in React Native releases.

Changelog:

[GENERAL] [FIXED] - Fix "Cannot read property 'canonical' of undefined" crash in Fabric event dispatch when a touch is delivered to a view unmounted mid-gesture

Test Plan:

• Verified via code inspection that undefined cannot originate from the reconciler
  itself (FiberNode constructor, detachFiberAfterEffects, and every stateNode
  assignment in completeWork only ever produce an object or null), so the loosened
  check only affects out-of-contract targets coming from native dispatch — for which
  every sibling guard in this pipeline already uses loose null checks.
• The change is limited to the stateNode guard in the two inlined copies of
  getListener (responder plugin + plugin registry) in each of the three generated
  bundles; the subsequent props guard is untouched. yarn jest react-native passes.
• Reproduction scenario: start a touch on a pressable row and unmount it before the
  event is processed (e.g. navigation triggered by the same interaction, or a recycled
  list removing the row). Before this change the dispatch throws the TypeError above;
  after it, the stale touch is ignored.