SheetModal: Fix - Dismiss bottom sheet on background instead of only clearing its content

[cmgustavo]

RN-2975

[JohnathanWhite]

The mechanism described here is real — I traced it. Clearing isModalVisible in the background handler short-circuits the !isVisible && isModalVisible guard in the effect below, so the imperative dismiss() never runs and the native sheet stays presented while React thinks it's gone. Skipping that state clear for bottom-sheet is the right shape of fix.

The new spec is a true regression test: with the fix reverted, dismisses the sheet when backgrounded, not just its content fails with 0 dismiss calls instead of 1. Both tests pass with the fix in.

One gap worth a follow-up (not a blocker for this PR). After this change, native dismissal depends entirely on the parent flipping isVisible in response to onBackdropPress. BottomNotification deliberately makes that callback a no-op when enableBackdropDismiss is false:

const handleBackdropPress = useCallback(() => {
  if (enableBackdropDismiss) {
    dispatch(AppActions.dismissBottomNotificationModal());
    ...
  }
}, [enableBackdropDismiss, dispatch, onBackdropDismiss]);

So for those callers — Passkeys.tsx:230, Theme.tsx:54, ThorswapDetails.tsx:224 — backgrounding the app now changes nothing at all and the sheet stays presented. I wrote a throwaway test for that case and got 0 dismiss() calls both before and after this patch, so it's a pre-existing gap rather than something this PR breaks. Worth its own ticket: if the point of the background path is keeping sheet content out of the app-switcher snapshot, the component should dismiss imperatively on background instead of trusting the caller to flip the controlled prop.

Related, also pre-existing: when Gorhom dismisses on its own, handleDismiss doesn't sync isModalVisible back to false, so the two can disagree if a caller leaves isVisible true.

Verified and fine:

• Dependency array is complete — isVisible, fullscreen, onBackdropPress, modalLibrary are all there, and the effect re-subscribes the AppState listener on change, so no stale closure.
• The react-native-modal branch and fullscreen sheets are genuinely untouched.
• Rapid close/reopen still works — the isDismissingRef / pendingOpenRef queueing is undisturbed.
• Spec hygiene is fine: restoreAllMocks covers both the prototype spies and the AppState.addEventListener spy, and the rerender/tree self-reference is lazily evaluated so it's valid.

One note on the test's reach: it spies on the @gorhom/bottom-sheet stub from test/setup.js, not the real class. That's the right call for asserting SheetModal's imperative calls, but it validates nothing about native lifecycle or unmount-while-backgrounded.

LGTM with the follow-up filed separately.