Ensure ClaimsPrincipal.Current is set if user changes

[twsouthwick]

The existing setup would only set Current to the existing user. Some scenarios seem to invoke at other times, so this uses the IRequestUserFeature to ensure that any updates to the user will be mirrored to the ClaimsUser.Current

[Copilot]

Pull Request Overview

This PR updates the System.Web Adapters to ensure ClaimsPrincipal.Current is properly synchronized with user changes throughout the request lifecycle. The implementation replaces the previous approach of setting Current only at request initialization with a feature-based system that mirrors any user updates to ClaimsPrincipal.Current.

Key changes:

• Introduced IRequestUserFeature.EnableStaticAccessors() method and SystemWebAdaptersOptions.EnableStaticUserAccessors property for opt-in static accessor management
• Refactored middleware architecture by splitting RegisterAdapterFeaturesMiddleware into separate RequestUserFeaturesMiddleware, RequestFeaturesMiddleware, and ResponseFeaturesMiddleware components
• Added CurrentPrincipalMiddleware to handle endpoint-specific static accessor enabling via SetThreadCurrentPrincipalAttribute

Reviewed Changes

Copilot reviewed 23 out of 24 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Microsoft.AspNetCore.SystemWebAdapters/SystemWebAdaptersOptions.cs	Adds configuration option for enabling static user accessors
src/Microsoft.AspNetCore.SystemWebAdapters/Adapters/Features/IRequestUserFeature.cs	Adds method to enable static accessor synchronization
src/Microsoft.AspNetCore.SystemWebAdapters/HttpContext.cs	Updates user setter to use required feature instead of extension method
src/Microsoft.AspNetCore.SystemWebAdapters/HttpRequest.cs	Updates LogonUserIdentity to use required feature directly
src/Microsoft.AspNetCore.SystemWebAdapters/Internal/RequestUserExtensions.cs	File deleted - functionality moved to middleware
src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/SystemWebAdaptersExtensions.cs	Adds extension method for configuring static accessors and updates middleware registration
src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/Middleware/RequestUserFeaturesMiddleware.cs	New middleware implementing user feature with static accessor synchronization
src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/Middleware/RequestFeaturesMiddleware.cs	New middleware handling request-related features
src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/Middleware/ResponseFeaturesMiddleware.cs	New middleware handling response-related features
src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/Middleware/CurrentPrincipalMiddleware.cs	New middleware enabling static accessors based on endpoint metadata
src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/RegisterAdapterFeaturesMiddleware.cs	File deleted - split into separate middleware components
src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/CurrentPrincipalMiddleware.cs	File deleted - replaced by new middleware implementation
Multiple middleware files	Namespace updated from Microsoft.AspNetCore.SystemWebAdapters to Microsoft.AspNetCore.SystemWebAdapters.Middleware
test/Microsoft.AspNetCore.SystemWebAdapters.Tests/HttpContextTests.cs	Adds test feature setup to ensure proper user feature behavior in tests
test/Microsoft.AspNetCore.SystemWebAdapters.E2E.Tests/RemoteAuthIdentityTests.cs	Adds validation for ClaimsPrincipal.Current behavior and refactors endpoint handling
test/Microsoft.AspNetCore.SystemWebAdapters.CoreServices.Tests/HttpRuntimeIntegrationTests.cs	Migrates tests from host builder to WebApplication pattern
test/Microsoft.AspNetCore.SystemWebAdapters.CoreServices.Tests/SelfHostedTestBase.cs	File deleted - test base class no longer needed
samples/AuthRemoteIdentity/AuthRemoteIdentityCore/Program.cs	Adds endpoint demonstrating ClaimsPrincipal.Current usage and corrects middleware order

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