Deprecate IActionContextAccessor

[Copilot]

With the introduction of endpoint routing, IActionContextAccessor is no longer necessary as developers can access action descriptor and metadata information directly through HttpContext.GetEndpoint(). This change deprecates both the interface and its implementation to guide developers toward the modern approach.

Changes Made

Added Obsolete Attributes

• Added [Obsolete] attribute to IActionContextAccessor interface with guidance message
• Added [Obsolete] attribute to ActionContextAccessor class with same guidance message
• Obsolete message: "IActionContextAccessor is obsolete. Use IHttpContextAccessor instead and access the endpoint information from HttpContext.GetEndpoint(). This type will be removed in a future version."

Maintained Backward Compatibility

• Added #pragma warning disable CS0618 around all internal framework usage to suppress warnings
• Framework continues to use the deprecated APIs internally to maintain compatibility
• All test code and test websites continue to function without build warnings

Migration Path

Developers should migrate from:

// Old approach
public class MyService
{
    private readonly IActionContextAccessor _accessor;
    
    public MyService(IActionContextAccessor accessor)
    {
        _accessor = accessor;
    }
    
    public void DoSomething()
    {
        var actionDescriptor = _accessor.ActionContext.ActionDescriptor;
        // ...
    }
}

To:

// New approach  
public class MyService
{
    private readonly IHttpContextAccessor _httpContextAccessor;
    
    public MyService(IHttpContextAccessor httpContextAccessor)
    {
        _httpContextAccessor = httpContextAccessor;
    }
    
    public void DoSomething()
    {
        var endpoint = _httpContextAccessor.HttpContext.GetEndpoint();
        var actionDescriptor = endpoint?.Metadata.GetMetadata<ActionDescriptor>();
        // ...
    }
}

Impact

• External users: Will receive CS0618 deprecation warnings with clear migration guidance
• Framework: Continues to function without changes, maintaining backward compatibility
• Tests: All existing functionality validated and continues to work

This deprecation aligns with ASP.NET Core's architectural evolution toward endpoint routing and provides a cleaner, more direct way to access endpoint metadata.

Fixes #28162.

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