Registration flag for all middleware

[B1ON1C]

Summary

Add a registration flag for all middleware to indicate whether it has already been added to the pipeline, similar to UseAuthentication and UseAuthorization. Currently, UseSession and some other middleware do not expose such a flag, making it hard to prevent duplicate registrations.

Motivation and goals

• Prevent accidental multiple registrations of the same middleware, which can lead to subtle bugs, duplicated processing, or session inconsistencies.
• Provide developers a consistent way to check if a middleware has already been added, improving safety and maintainability.
• UseAuthentication and UseAuthorization already implement this pattern with internal flags, but UseSession does not. Adding this would unify middleware behavior across the framework.

Goals:

• Introduce a standard mechanism for middleware to track registration.
• Ensure existing middleware like UseSession, UseCors, UseResponseCaching can optionally adopt this.
• Maintain backward compatibility with existing applications.

In scope

• Adding an internal registration flag to SessionMiddleware and its extension methods. (UseCors, UseResponseCaching...)

Out of scope

• Changing the public API signature of middleware extension methods.

Risks / unknowns

• Developers may expect UseSession() to be idempotent already; adding the flag could change subtle behaviors if a middleware was previously registered twice intentionally.
• Backward compatibility must be preserved: applications that currently register UseSession multiple times should not break.
• Security/performance: minimal, but double registration previously could have caused slight overhead or unintended side effects.

Examples

```csharp
// Program.cs
app.UseSession();

// Inside a NuGet package or shared library
if (!app.Properties.ContainsKey("SessionRegistered"))
{
    app.UseSession();
}

// Developers can rely on similar pattern to UseAuthorization/UseAuthentication