[material_ui] Fix MenuAnchor scrollbar safe-area padding

[abdullahtas0]

Material Scrollbar uses the surrounding MediaQuery.padding as painter padding when no explicit padding is provided. A MenuAnchor overlay is already positioned within the safe area, so inheriting the view padding again shortens the scrollbar track and prevents the thumb from reaching its visual end.

This change removes safe-area padding only for the menu's Scrollbar, then restores the original MediaQueryData below it so custom menu children continue to observe the same media query values as before. It also adds a regression test and a patch changelog entry.

Fixes flutter/flutter#188155.

This supersedes flutter/flutter#191085, which cannot land in flutter/flutter because Material framework changes have moved to material_ui.

Validation:

• The regression test failed before the source change with a 24 logical pixel bottom scrollbar padding and passes after the change.
• flutter test test/menu_anchor_test.dart (171 passed, 1 skipped)
• flutter analyze --no-pub
• flutter_plugin_tools format --packages=material_ui
• flutter_plugin_tools validate --packages=material_ui --check-for-missing-changes

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• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Code Review

This pull request prevents MenuAnchor scrollbars from inheriting system safe-area padding by removing padding from the MediaQuery surrounding the Scrollbar and restoring it for the inner scroll view. A regression test and a changelog entry are also included. The feedback suggests optimizing the implementation by using mediaQuery.removePadding(...) directly on the already retrieved MediaQueryData instance, avoiding redundant MediaQuery.of(context) lookups.