Fix Blazor root component state persistence across render modes

[Copilot]

Summary

This PR fixes state persistence for Blazor root components where the @key directive information was lost when transitioning between prerendering and interactive modes, causing different persistence keys to be generated.

Problem

The issue occurred because GetSerializableKey in SupplyParameterFromPersistentComponentStateValueProvider produced different values when rendering statically vs. interactively. During prerendering, the parent component hierarchy differed from interactive mode, leading to inconsistent key generation and failed state persistence for root components.

Solution

Core Changes

1. Added ComponentState.GetComponentKey() - New protected virtual method that extracts @key directives from the parent component's render tree, providing a consistent interface across render modes.

2. Enhanced EndpointComponentState - Overrides GetComponentKey() to detect SSRRenderModeBoundary components and return the ComponentMarkerKey for consistent root component identification.

3. Specialized ComponentState subclasses - Created WebAssemblyComponentState and RemoteComponentState that accept ComponentMarkerKey parameters and return them from GetComponentKey() when available.

4. Improved key computation - Moved ComputeKey logic to ComponentState with enhanced parent type filtering that excludes SSRRenderModeBoundary components from the hierarchy calculation.

5. SSRRenderModeBoundary helper - Added GetComponentMarkerKey() method to expose the internal marker key for state persistence.

Key Algorithm

The solution ensures consistent key generation by:

• Using the same ComputeKey algorithm across all render modes
• Providing ComponentMarkerKey consistently for root components
• Filtering out SSRRenderModeBoundary from parent type calculations
• Maintaining serializable key validation

Example

Before this fix, a root component with state persistence would fail:

@rendermode InteractiveServer
<div>
    <p>Counter: @count</p>
    <button @onclick="Increment">Increment</button>
</div>

@code {
    [SupplyParameterFromPersistentComponentState]
    private int count { get; set; } = 0;

    private void Increment() => count++;
}

The count value would be lost during prerender-to-interactive transitions because different keys were generated in each mode. With this fix, the same key is used consistently, preserving the state.

Testing

• Updated all existing tests to use the new ComponentState.ComputeKey() API
• Added comprehensive validation for cross-render-mode key consistency
• Maintained backward compatibility for the key generation algorithm
• Verified proper ComponentMarkerKey integration

Fixes #62331.

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[javiercn]

/azp run Build Analysis

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[javiercn]

/backport to release/10.0-preview6

[github-actions]

Started backporting to release/10.0-preview6: https://github.com/dotnet/aspnetcore/actions/runs/15737443923