[material_ui] Fix DropdownMenuFormField missing leading icon

[haashem]

Fixes a regression where DropdownMenuFormField.leadingIcon was not shown in the field.

DropdownMenuFormField took a leadingIcon, but didn’t actually show it in the field. This change makes it behave like trailingIcon, so the leading icon now shows up as the default prefix icon.

Fixes flutter/flutter#185416

With only leadingIcon

decorationBuilder has higher priority than leading icon.

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[gemini-code-assist]

Code Review

This pull request ensures that DropdownMenuFormField.leadingIcon is displayed in the text field by assigning it as the prefix icon when InputDecoration.prefixIcon is null. It also adds documentation and a widget test to verify this behavior. The feedback suggests optimizing performance by checking if widget.leadingIcon is non-null before calling copyWith to avoid unnecessary InputDecoration allocations.