[iOS] Preserve semantics parents after reparenting (#189686)

Fixes https://github.com/flutter/flutter/issues/100946.

## Problem

Reordering a `ReorderableListView` on iOS could move VoiceOver focus to
the upper-left corner of the screen. The affected items could no longer
be focused at their visual positions.

During a reorder, semantics children move through temporary containers
used by the drag overlay and drop animation. A child may already have
been attached to its replacement `SemanticsObject` when its previous
parent is updated or deallocated.

The previous parent unconditionally cleared the child's `parent`
property. This overwrote the new parent relationship and disconnected
the child from the native semantics hierarchy.

`SemanticsObject` calculates its global accessibility frame by walking
this parent chain and composing each ancestor's transform. Once the
chain was disconnected, the element retained only its local
physical-pixel rectangle. On a 3x simulator, the expected frame:

```
{{0, 286}, {402, 56}}
```

became:

```
{{0, 0}, {134, 18.67}}
```

This accounts for both the missing translation and the frame being
reduced by the device pixel ratio.

## Fix

Only clear a child's parent when the `SemanticsObject` performing
cleanup still owns that parent relationship:

```objc
child.parent == self
```

This guard is applied when:

- A `SemanticsObject` is deallocated.
- Its traversal-order children are replaced.
- Its hit-test-order children are replaced.

A previous parent can now release its stale child references without
detaching a child that has already moved under a replacement container.
The complete ancestor transform remains available when calculating the
native accessibility frame.

## Testing

Added An iOS XCUITest that performs the default mobile long-press
reorder gesture, verifies the resulting native semantic traversal order,
and asserts that the reordered item retains the expected accessibility
frame.

The integration app also includes a deterministic direct-drag fixture
for exercising XCUITest drag synthesis.

Validation performed:

- `testReorderableListAccessibilityHierarchyUpdates` passes with the
locally built engine.
- `packages/flutter/test/widgets/reorderable_list_test.dart` passes.
- `packages/flutter/test/material/reorderable_list_test.dart` passes.
- `ios_test_flutter` builds successfully.
- Engine formatting and Dart analysis pass.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

---------

Co-authored-by: LongCatIsLooong <31859944+LongCatIsLooong@users.noreply.github.com>