Add checkbox model binding integration tests

[Copilot]

Adds integration tests for checkbox tag helper model binding, similar to existing tests for text input, select, and other form elements.

Changes

• Test coverage: Added Checkboxes_RendersCorrectly test validating name/ID generation, checked state from model, and metadata binding (legend, hint, error message)
• Test infrastructure: Added CheckboxesTestsModel with List<string> property, controller action, and Checkboxes.cshtml view

Bug fixes discovered during implementation

• CheckboxesItemTagHelper: Fixed checked state determination when ModelState contains errors. Previously used ModelState.RawValue (null for errors) instead of falling back to model values:

// Before: Always used ModelState.RawValue when entry exists (null for errors)
var value = ViewContext.ModelState.TryGetValue(modelExpression.Name, out var entry) ?
    entry.RawValue : model;

// After: Only use ModelState.RawValue if it's a valid enumerable collection
var value = ViewContext.ModelState.TryGetValue(modelExpression.Name, out var entry) &&
    entry.RawValue is IEnumerable and not string ?
    entry.RawValue : model;

• CheckboxesContext/RadiosContext: Fixed SetErrorMessage() validation to permit error messages inside open fieldsets (consistent with SetHint() behavior and GDS patterns)

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