[material_ui] Fix ExpansionTile nested WidgetSpan semantics assertion

[ahamSel]

Superseded by #12765, which contains the rebased fix and the simpler test requested in flutter/flutter#190639. This PR remains closed because GitHub cannot reopen it after the branch was rebased. The description and validation below refer to this PR's original diff.

On Android, ExpansionTile wraps its tile in a live-region Semantics node that uses AccessibilityFocusBlockType.blockNode. When the title contains a nested WidgetSpan, that live region can merge into the surrounding semantics tree and trigger a sibling-configuration conflict assertion.

This makes the live region a semantics container, so blockNode applies to an explicit node. It also adds an Android regression test using the reported nested WidgetSpan structure. The test fails with the original !conflict assertion when the production change is removed and passes with the fix.

Fixes flutter/flutter#188298.

This ports flutter/flutter#190639 to material_ui, where Material development is moving during the framework code freeze.

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Validation performed:

• flutter test test/expansion_tile_test.dart
• flutter analyze lib/src/expansion_tile.dart test/expansion_tile_test.dart

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

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Footnotes

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[Piinks]

Hi @ahamSel thanks for contributing! This package is not accepting contributions yet, so I am going to close this PR for now. We will be letting folks know when we are ready for contributions, the PR you have open in flutter/flutter is perfect. We'll be letting everyone know there when we are ready. Thanks!