Clarify IExceptionHandler middleware requirements

[Copilot]

The IExceptionHandler docs implied that registering handlers with AddExceptionHandler<T>() was enough. They also omitted key runtime behaviors around bare UseExceptionHandler(), handled-exception logging in .NET 8/9, and 404 responses from handlers that don't write a response.

• Middleware setup
  • Added an important note that registered IExceptionHandler implementations only run when exception handling middleware is added.
  • Updated the registration example to show both AddExceptionHandler<T>() and UseExceptionHandler(...).

builder.Services.AddExceptionHandler<CustomExceptionHandler>();

// ...

app.UseExceptionHandler("/Error");

• UseExceptionHandler configuration

  • Added guidance that parameterless UseExceptionHandler() requires a fallback through an error path, problem details, or a fallback handler.
  • Clarified that registered handlers run before the configured fallback and the fallback is used only when all handlers return false.

• Handled exception behavior

  • Documented that handlers returning true stop later handlers and must write the full response.
  • Added the 404/AllowStatusCode404Response warning for handlers that return true without setting a response.

• Version-specific diagnostics

  • Added .NET 8/9 guidance that middleware diagnostics are still emitted for handled exceptions and showed how to filter the middleware log category.
  • Left .NET 10 guidance aligned with the existing diagnostics suppression section.

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Internal previews

File	Preview link
aspnetcore/fundamentals/error-handling.md	aspnetcore/fundamentals/error-handling
aspnetcore/fundamentals/error-handling/includes/error-handling8.md	aspnetcore/fundamentals/error-handling/includes/error-handling8
aspnetcore/fundamentals/error-handling/includes/error-handling9.md	aspnetcore/fundamentals/error-handling/includes/error-handling9