[dev-v5] fix: change does not trigger validation in `FluentSelect`

[MarvinKlein1508]

🐛 Bug Report

When switching the value in FluentSelect then the validation for the current bound value will not retrigger.

Please take a look on this video:

2026-04-20.10-22-11.mp4

💻 Repro or Code Sample

@page "/Debug/Test"
@layout FluentUI.Demo.Client.Layout.EmptyLayout


<div style="padding: 1rem">
    <FluentButton OnClick="Save">Submit</FluentButton>
    <EditForm @ref="_form" Model="Input" OnValidSubmit="OnValidSubmit">
        <DataAnnotationsValidator />
        <FluentValidationSummary />
        <FluentSelect Label="Location"
                      TOption="CompanyLocation"
                      TValue="int?"
                      @bind-Value="@Input.LocationId">
            <FluentOption TValue="int?" Value="@(null)" Text="--- SELECT ---" />
            @foreach (var item in Locations)
            {
                <FluentOption TValue="int?" Value="@item.Id">
                    @item.Name
                </FluentOption>
            }
        </FluentSelect>
        <ValidationMessage For="() => Input.LocationId" />

        <div>
            <label>Location 2</label>
            <InputSelect @bind-Value="@Input.LocationId2">
                <option value="">--- SELECT ---</option>
                @foreach (var item in Locations)
                {
                    <option value="@item.Id">
                        @item.Name
                    </option>
                }
            </InputSelect>
            <ValidationMessage For="() => Input.LocationId2" />
        </div>
    </EditForm>
</div>

@code {
    private EditForm _form = default!;
    public MyInput Input { get; set; } = new();

    Task Save()
    {
        _form.EditContext!.Validate();

        return Task.CompletedTask;
    }

    Task OnValidSubmit()
    {
        return Task.CompletedTask;
    }

    public IEnumerable<CompanyLocation> Locations { get; set; } = new List<CompanyLocation>
    {
        new CompanyLocation { Id = 1, Name = "Germany" },
        new CompanyLocation { Id = 2, Name = "Netherlands" },
        new CompanyLocation { Id = 3, Name = "Belgium" }
    };

    public class MyInput
    {
        [System.ComponentModel.DataAnnotations.Required]
        public int? LocationId { get; set; }

        [System.ComponentModel.DataAnnotations.Required]
        public int? LocationId2 { get; set; }
    }

    public class CompanyLocation
    {
        public int Id { get; set; }
        public required string Name { get; set; }
    }
}

1. Open the page
2. Click submit
3. Notice two validation errors
4. Switch the value in the native control
5. See validation message is disappearing
6. Switch the value in FluentSelect
7. Notice the validation message is still there.

🤔 Expected Behavior

I expect the FluentSelect to behave exactly like InputSelect in this case.

💁 Possible Solution

🔦 Context

🌍 Your Environment

• OS & Device: Windows 11 64 Bit
• Browser latest Firefox, Chrome & Edge
• latest dev-v5 branch

[dvoituron]

Probably related to #4472