Fix ExpansionTile semantics with WidgetSpan titles - #190639

On Android, embedding an `ExpansionTile` in a `WidgetSpan` can trigger a sibling-configuration conflict assertion when semantics are enabled. Its live-region `Semantics` uses `AccessibilityFocusBlockType.blockNode` but can merge into the surrounding semantics tree.

This makes the live region a semantics container, so `blockNode` applies to an explicit node. The regression test uses the simpler reproduction suggested in flutter/flutter#190639 and relies on the semantics handle managed by `testWidgets`.

Fixes flutter/flutter#188298.

This ports flutter/flutter#190639 to `packages/material_ui`. The earlier package PR, #12383, was closed while contributions were paused. On October 2, I rebased this replacement onto current `main` (`10cde9a3b`) to refresh its stale CI configuration. The contribution itself is unchanged: the semantics fix, its regression test, and a pending changelog entry. There are no custom CI changes in this PR.

I used AI to help with the fix and regression test.

## Pre-Review Checklist

Local validation after the October 2 rebase, on macOS with the already-installed Flutter 3.47.5 / Dart 3.13.4:

- Full `material_ui` suite: `flutter test --no-pub --concurrency=2` — 8,222 passed, 12 existing skips, no failures.
- Full example suite: `flutter test --no-pub --concurrency=1` in `packages/material_ui/example` — 406 passed, two existing skips, no failures.
- ExpansionTile file separately: 64 passed, one existing skip (also included in the full suite).
- Defaults generator suite: `dart test` — 72 passed.
- Full-package `flutter analyze --no-pub`: no issues.
- Dart formatting and `git diff --check`: passed.
- `dart run script/tool/bin/flutter_plugin_tools.dart validate --packages material_ui --check-for-missing-changes --base-branch upstream/main`: passed.

I repeated the regression's fail-without-fix check on the rebased branch: removing `container: true` produces the original `!conflict` assertion; restoring it makes the test pass. The existing `flutter_goldens` harness skips golden-image comparisons; I did not change that behavior. These are local test results, not a claim that repository-wide CI has passed.

GitHub automatically removed the `CICD` label after the October 2 push. Could a maintainer reapply it for the refreshed head so the repository presubmits can run?

The release metadata uses `material_ui`'s batch-release convention: a pending changelog entry with `version: patch`.

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