Add behavior tests for BitSelectField

[Copilot]

Adds behavior tests to verify value binding and type conversion for the BitSelectField component, addressing missing test coverage for the component's core functionality.

Tests Added

Created BitSelectFieldTest.Behaviors.cs with 8 test methods covering:

• Type conversion: String, int, and enum value changes
• Nullable types: Nullable int and enum handling
• Two-way binding: Bound value updates on selection changes
• DOM state: Selected attribute setting and updates

[Fact]
public void BitSelectField_Should_Change_Enum_Value_Correctly()
{
    TestEnum value = TestEnum.Option1;
    
    var component = ctx.Render<BitSelectField<TestEnum>>(parameters => parameters
        .Bind(p => p.Value, value, v => value = v)
        .AddChildContent<BitSelectItem<TestEnum>>(/* ... */));
    
    var select = component.Find("select");
    select.Change(TestEnum.Option3);
    
    Assert.Equal(TestEnum.Option3, value);
}

All tests pass with no warnings or security alerts.

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