feat(blazor): add CssClass parameter to ValidationSummary component

[EduardF1]

Summary

Fixes #43860

Adds a CssClass parameter to the ValidationSummary Blazor component so users can pass custom CSS classes that get appended to the default validation-errors class.

Before:

<ValidationSummary class="pt-2" />
<!-- renders: class="validation-errors"  (class attribute ignored) -->

After:

<ValidationSummary CssClass="pt-2" />
<!-- renders: class="validation-errors pt-2" -->

Implementation

Implements the approach suggested by @SteveSandersonMS in the issue comments. Adds a CssClass parameter following the same pattern used by InputBase<T> and other Blazor form components:

• AttributeUtilities.CombineClassNames merges AdditionalAttributes["class"] (if any), the base validation-errors class, and the new CssClass parameter
• The render tree order is swapped: AddMultipleAttributes fires first, then AddAttribute for class so the computed combined class correctly overrides any class key that might exist in AdditionalAttributes

Changes

• src/Components/Web/src/Forms/ValidationSummary.cs — adds CssClass parameter and updates BuildRenderTree
• src/Components/Web/src/PublicAPI.Unshipped.txt — new public API entries
• src/Components/test/testassets/BasicTestApp/FormsTest/TypicalValidationComponent.razor — adds a second <ValidationSummary CssClass="custom-summary-css-class" /> for E2E coverage
• src/Components/test/E2ETest/Tests/FormsTest.cs — adds ValidationSummaryAppliesCssClass E2E test

[dotnet-policy-service]

Thanks for your PR, @EduardF1. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[EduardF1]

@dotnet-policy-service agree