[Blazor] Add WithStaticAssets extension for IEndpointConventionBuilder

[Copilot]

Adds a new WithStaticAssets extension method that works with any IEndpointConventionBuilder, enabling endpoints that return RazorComponentResult and other custom endpoints to attach ResourceCollection metadata.

Problem

Previously, the WithStaticAssets method was only available on RazorComponentsEndpointConventionBuilder, limiting its use to Blazor component endpoints. This created a gap for scenarios where developers wanted to use RazorComponentResult in regular endpoints (like MapGet, MapPost, etc.) but still needed the same static asset metadata that Blazor component endpoints receive automatically.

Solution

This PR introduces EndpointConventionBuilderResourceCollectionExtensions.WithStaticAssets<TBuilder>() that:

• Extends any IEndpointConventionBuilder to attach ResourceCollection metadata
• Takes an IEndpointRouteBuilder parameter to resolve static assets from
• Supports optional manifest path parameter for custom manifests
• Automatically prevents duplicate metadata by checking for existing ResourceAssetCollection
• Uses the same underlying infrastructure (ResourceCollectionResolver) as the existing Blazor-specific implementation

Usage

var app = WebApplication.CreateBuilder().Build();
app.MapStaticAssets();

// Now any endpoint can have static asset metadata
app.MapGet("/page/{name}", (string name) => 
   new RazorComponentResult<MyComponent>(new { Name = name }))
   .WithStaticAssets(app);

app.MapPost("/api/render", (RenderRequest request) => 
   new RazorComponentResult<DynamicComponent>(request.Parameters))
   .WithStaticAssets(app, "custom-manifest.json");

Implementation Details

The new extension method:

1. Creates a ResourceCollectionResolver from the provided IEndpointRouteBuilder
2. Adds a convention that checks for existing metadata to avoid duplicates
3. Resolves and attaches the same metadata types as the Blazor version: ResourceAssetCollection, ResourcePreloadCollection, and ImportMapDefinition
4. Only applies metadata when static assets are actually registered for the specified manifest

This maintains full compatibility with existing code while extending the functionality to work with any endpoint convention builder.

Fixes #58937

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Blazor] Provide a helper to attach ResourceCollection metadata to other endpoints</issue_title>
<issue_description>This will be useful in the context of RazorComponentResult

#58937 (comment)</issue_description>

<agent_instructions>We want to provide a helper like that doesn't use RazorComponentsEndpointConventionBuilder but instead just extends IEndpointRouteBuilder

aspnetcore/src/Components/Endpoints/src/Builder/RazorComponentsEndpointConventionBuilderExtensions.cs

Lines 50 to 64 in 063ba34

	public static RazorComponentsEndpointConventionBuilder WithStaticAssets(
	this RazorComponentsEndpointConventionBuilder builder,
	string? manifestPath = null)
	{
	ArgumentNullException.ThrowIfNull(builder);
	builder.ManifestPath = manifestPath;
	if (!builder.ResourceCollectionConventionRegistered)
	{
	builder.ResourceCollectionConventionRegistered = true;
	var convention = new ResourceCollectionConvention(new ResourceCollectionResolver(builder.EndpointRouteBuilder));
	builder.BeforeCreateEndpoints += convention.OnBeforeCreateEndpoints;
	builder.Add(convention.ApplyConvention);
	}
	return builder;

It can be simplified to only do the following. If not done, it should check there isn't a resource collection already on the metadata.

            var convention = new ResourceCollectionConvention(new ResourceCollectionResolver(builder.EndpointRouteBuilder));
            builder.Add(convention.ApplyConvention);
```</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

Fixes #63398

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