Support NotFound content rendering for a custom Router

[ilonatommy]

This PR:

• Removes support for NotFound fragment - it cannot be rendered using NavigationManager.NotFound(). UPDATE DOCS
• Fixes issues with raising NotFound event after async operations.
• Adds support for raising NotFound event without blazor default Router component.

Implementation description

• Moves handling 404 signal from endpoint renderer to the endpoint invoker.
• Refactors one SetNotFoundResponseAsync with httpContext.Response.HasStarted condition into 2 separate methods that are called before and after response started, if 404 event was triggered.
• Communicates the the renderer that Router handled rendering of NotFoundPage by setting the Path in NotFoundEventArgs. If that property is not set, router treats the request as not handled, cleans the response and avoids flushing in order to allow re-execution middleware act.
• Clean up tests and apply changes from Endpoint renderer waits for quiescence with stopping the rendering #62524 to support testing triggering 404 after async operation.

Fixes #62367, Fixes #62469, Fixes #62153

[Copilot]

Pull Request Overview

This PR refactors the NotFound event handling in Blazor components to better support custom routers and fix issues with async operations. The main changes include removing the NotFound fragment support, moving 404 handling from the endpoint renderer to the endpoint invoker, and improving the communication between routers and renderers about NotFound event handling.

Key Changes:

• Moves NotFound event handling from endpoint renderer to endpoint invoker for better async operation support
• Refactors NotFoundEventArgs to use a settable Path property to indicate router handling
• Updates test infrastructure to better test async NotFound scenarios and custom router behavior

Reviewed Changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
NotFoundEventArgs.cs	Converts constructor parameter to settable property for router communication
Router.cs	Updates to set Path property when handling NotFound events
NavigationManager.cs	Simplifies NotFound event triggering with reusable event args
RazorComponentEndpointInvoker.cs	Moves NotFound handling logic from renderer to invoker
EndpointHtmlRenderer.cs	Refactors NotFound response handling into separate methods
CustomRouter.cs	Adds new custom router implementation for testing
Test files	Updates test infrastructure and parameter names for better async testing