[material_ui] Port flutter/flutter #185475 "Accessibility: Add semanticLabel to MenuAnchor"

[mbcorona]

Ports flutter/flutter#185475 to material_ui, following the instructions in flutter/flutter#188444. The commits were cherry-picked from the original PR, with only file paths adjusted for this repo.

MenuAnchor had no way to label the expanded menu overlay, so screen readers announced nothing useful when a menu opened. This adds a semanticLabel property, applied to a Semantics node wrapping the submenu in _buildOverlay, and only when a label is provided.

The original PR also announced the label with SemanticsService.announce when the menu opened. Following @chunhtai's review feedback on flutter/flutter#185475, that part is dropped here: a semantics label should not double as announcement text, and anyone who wants an announcement can call SemanticsService.announce from onOpen. The last commit on this branch removes it.

Tests

New test MenuAnchor applies semanticLabel to the expanded menu overlay in packages/material_ui/test/menu_anchor_test.dart, checking that the label is absent before the menu opens and present once it expands. The menu_anchor.0 API example now sets a semanticLabel, and its example test still passes.

Fixes flutter/flutter#184876

Supersedes flutter/flutter#185475, which will be closed with a pointer to this PR.

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Code Review

This pull request introduces a semanticLabel property to the MenuAnchor widget, wrapping the expanded menu overlay in a Semantics widget when a label is provided. A corresponding test case and example update are also included. The review feedback recommends setting explicitChildNodes: true on the Semantics widget to ensure child menu items are treated as separate interactive nodes in the semantics tree.