fix(iOS): gate canBecomeFirstResponder behind enableImperativeFocus flag (#56366)

Summary:
Fixes a critical iOS regression introduced in RN 0.83 (#53825) where `RCTViewComponentView.canBecomeFirstResponder` unconditionally returns `YES`. This causes UIKit to promote parent views to first responder via `_promoteSelfOrDescendantToFirstResponderIfNecessary`, resulting in:

1. **TextInput autoFocus regression** — inputs lose focus on mount (focus→blur cycle)
2. **Keyboard reopening** — after any TextInput was focused/dismissed, opening native overlays (UIMenu, dropdown menus, header toolbar buttons) causes the keyboard to reappear unexpectedly

### Root Cause

PR #53825 added `canBecomeFirstResponder: YES` as part of the `enableImperativeFocus` feature, but didn't gate the return value behind the feature flag. When the flag is `false` (default), every `RCTViewComponentView` still claims first responder eligibility, causing UIKit's `_promoteSelfOrDescendantToFirstResponderIfNecessary` to steal focus from TextInputs.

Two distinct failure modes:

1. **On navigation transitions** — UIKit calls `_promoteSelfOrDescendantToFirstResponderIfNecessary` on the incoming screen's root view, walks the hierarchy, finds an `RCTViewComponentView` that returns `YES`, and promotes it — stealing focus from a TextInput that just called `becomeFirstResponder`.

2. **On native overlay dismissal** — When UIMenu/context menu/toolbar dismisses, UIKit restores first responder by walking to the nearest view claiming first responder eligibility, which is now every `RCTViewComponentView`.

### Fix

Gate `canBecomeFirstResponder` behind the `enableImperativeFocus` feature flag. When `false` (the default for all existing apps), views don't claim first responder — fixing the regression. When `true`, the imperative focus/blur feature works as designed.

This change is safe: `enableImperativeFocus` defaults to `false`, so the gated path only activates for apps explicitly opting in to the new focus/blur commands.

Credit to war-in for identifying this fix in #55908.

Fixes #55116
Related: expo/expo#43198, software-mansion/react-native-screens#3763, software-mansion/react-native-screens#3677

## Changelog:[iOS] [Fixed] - Gate RCTViewComponentView.canBecomeFirstResponder behind enableImperativeFocus flag to prevent keyboard reopening and focus theft in Fabric apps

Pull Request resolved: #56366

Test Plan:
1. Create a screen with a `TextInput` that has `autoFocus={true}`
2. Navigate to that screen using React Navigation (Stack)
3. Verify `TextInput` retains focus and keyboard stays visible — previously it would focus then immediately blur
4. Dismiss the keyboard, then open a native UIMenu or press a toolbar button
5. Verify keyboard does **NOT** reopen
6. Enable `enableImperativeFocus` flag, verify `focus()`/`blur()` commands still work on Views

> **Note:** This PR is intended as a backup to #55908 by war-in, which has the identical fix. If that PR gets reviewed and merged, this can be closed. Opening this to increase visibility on a critical regression affecting all RN 0.83+ iOS Fabric apps.

Reviewed By: cipolleschi

Differential Revision: D99994820

Pulled By: javache

fbshipit-source-id: bdfea2b01e43afa05e8cf35d12d9ccf7a56d63ca

Author: hknakn

packages/react-native/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm

@@ -1721,7 +1721,7 @@ - (void)transferVisualPropertiesFromView:(UIView *)sourceView toView:(UIView *)d
 
 - (BOOL)canBecomeFirstResponder
 {
-  return YES;
+  return ReactNativeFeatureFlags::enableImperativeFocus();
 }
 
 - (void)handleCommand:(const NSString *)commandName args:(const NSArray *)args