[material_ui] Add trailingIconVisibility to ExpansionPanel (#179167)

[benji-farquhar]

Ports flutter/flutter#183951 to material_ui, following the instructions in flutter/flutter#188444.

Description

Introduces the ExpansionPanelIconVisibility enum (visible, hidden, gone) to ExpansionPanel.trailingIconVisibility.

This allows controlling whether the trailing expand/collapse icon is visible, hidden (preserving its layout space for alignment), or gone (removed from layout completely).

Fixes flutter/flutter#179167
Supersedes flutter/flutter#183951

cc @dkwingsmt @victorsanni who previously reviewed and approved the original PR in flutter/flutter#183951.

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Code Review

This pull request introduces the ExpansionPanelIconVisibility enum and a corresponding trailingIconVisibility property to ExpansionPanel and ExpansionPanelRadio to control the visibility of the trailing expand/collapse icon. It also includes an example application, unit tests, and integration tests. The review feedback correctly identifies multiple instances where enum values are referenced using invalid leading dot shorthand (e.g., .visible, .hidden, .gone) instead of their fully qualified names, which will cause compilation errors in Dart.