[beta] [web] Cancel touch pointers WebKit abandons mid-gesture (#190231)

This pull request cherry-picks #189608 into the beta release branch (flutter-3.47-candidate.0) so the fix ships in the next stable release.

### Issue Link:
https://github.com/flutter/flutter/issues/188781

### Impact Description:
P0 regression. On iOS 27, which affects every browser since they all run WebKit, a text field in a Flutter web app can become permanently unresponsive after a common gesture. When the user long-press-drags the selection caret and
lifts, WebKit abandons the touch without delivering `pointerup` or `pointercancel`, leaving the framework holding a pointer that never lifts. This wedges the tap-and-drag gesture recognizer, so every later tap is silently dropped: the field never regains focus and the keyboard never returns, and the user can no longer type. It breaks text input in shipping production web apps. iOS 26 is unaffected.

### Changelog Description:
[flutter/188781] When dragging the text selection caret on iOS 27 web browsers, text fields could become permanently unresponsive to taps and never regain focus; they now recover.

### Workaround:
No in-app workaround. Reloading the page temporarily restores the field, but it breaks again the next time the gesture occurs.

### Risk:
The change is small and strictly gated to iOS WebKit. It only cancels touch pointers the browser has already dropped from its `touches` list, so it cannot affect other platforms or normal pointer handling.

### Test Coverage:
The fix adds 341 lines of engine unit tests in `pointer_binding_test.dart` covering the touch-cancel logic.

### Validation Steps:
1. Open a Flutter web app containing a TextField on an iOS 27 device, any browser.
2. Tap the field to focus it and type some text.
3. Long-press the selection caret, drag it, then lift your finger.
4. Tap the field again, and repeat a few times.
5. Before the fix the field becomes permanently dead: taps do nothing, no focus, no keyboard. After the fix the field keeps responding and recovers.

Demo before: https://flutter-demo-47-before.web.app
Demo after:  https://flutter-demo-47-after.web.app