.NET: Support ClaimsIdentity-based scoping of agent sessions

[lokitoth]

Motivation and Context

The current Hosting helpers rely on a single shared AgentSessionStore, relying on conversationId or contextId coming from the incoming request, regardless of authentication status. This makes it difficult to use in multi-user/tenant environments.

Description

• Add support for the decorator pattern over AgentSessionStore with DelegatingAgentSessionStore - analogous to DelegatingAIAgent -> AIAgent
• Implement UserIdentityScopedSessionStore, which will pick up the ambient ClaimsIdentity user from the ASP.Net HttpContext, grab the first claim matching a user-specified type, and use the value, if any, to scope the AgentSession to the identity.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• [ ] Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[github-actions]

Automated Code Review

Reviewers: 4 | Confidence: 81%

✓ Correctness

The PR introduces a well-structured decorator pattern for AgentSessionStore (DelegatingAgentSessionStore) and a concrete implementation that scopes sessions by user identity claims (UserIdentityScopedSessionStore). The code correctly follows the existing DelegatingAIAgent pattern, properly handles strict/non-strict modes, and all operations delegate correctly to the inner store with scoped conversation IDs. No correctness bugs found.

✓ Security Reliability

The PR introduces a well-structured decorator pattern for session stores with identity scoping. The security design is sound: strict mode defaults to true (secure by default), null checks are in place, and the delegation pattern is correctly implemented. One moderate defense-in-depth concern exists around the composite key format used for session scoping, where the separator character could theoretically allow key collisions between different user/conversationId combinations.

✓ Test Coverage

The test coverage for both DelegatingAgentSessionStore and UserIdentityScopedSessionStore is solid, covering constructor validation, delegation behavior, strict/non-strict modes, null HttpContext, custom claim types, and multi-user isolation. Two minor gaps exist: (1) UserIdentityScopedSessionStore tests never verify that the CancellationToken passed by the caller is forwarded to the inner store (all assertions use It.IsAny<CancellationToken>()), and (2) there is no test for a claim that exists but has an empty-string value, which would produce the same scoped key as the non-strict no-claim scenario, representing a potential silent collision.

✗ Design Approach

The main design issue is in the new non-strict path for UserIdentityScopedSessionStore: the docs say missing claims should "proceed without scoping," but the implementation and tests currently re-key those sessions by prepending an empty scope. That silently changes storage identity for anonymous or missing-claim requests instead of preserving the existing conversation key shape.

Flagged Issues

• UserIdentityScopedSessionStore documents non-strict mode as "operations proceed without scoping when the claim is absent" (lines 35-37), but the implementation and tests lock in ":{conversationId}" as the scoped key. Because downstream stores like InMemoryAgentSessionStore.GetKey already derive their own key prefix, this silently remaps missing-claim traffic from agentId:conversationId to agentId::conversationId instead of leaving it unscoped.

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Automated review by lokitoth's agents

[Copilot]

Pull request overview

This PR introduces a decorator foundation for .NET AgentSessionStore implementations and adds an ASP.NET Core-specific session store that scopes conversation/session keys by the current user’s claim from HttpContext, enabling per-user session isolation in multi-user/tenant hosting scenarios.

Changes:

• Added DelegatingAgentSessionStore as a decorator-style base for session store composition.
• Added UserIdentityScopedSessionStore (ASP.NET Core) to scope session keys by a selected claim from the ambient HttpContext user.
• Added unit tests and solution/project wiring for the new stores and new ASP.NET Core hosting project.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Hosting/DelegatingAgentSessionStore.cs	Introduces a delegating/decorator base AgentSessionStore implementation.
dotnet/src/Microsoft.Agents.AI.Hosting.AspNetCore/UserIdentityScopedSessionStore.cs	Adds an ASP.NET Core session store decorator that scopes keys using IHttpContextAccessor claims.
dotnet/src/Microsoft.Agents.AI.Hosting.AspNetCore/Microsoft.Agents.AI.Hosting.AspNetCore.csproj	New ASP.NET Core hosting project/package for the identity-scoped store.
dotnet/tests/Microsoft.Agents.AI.Hosting.UnitTests/DelegatingAgentSessionStoreTests.cs	Unit coverage for the delegating store pass-through behavior.
dotnet/tests/Microsoft.Agents.AI.Hosting.UnitTests/UserIdentityScopedSessionStoreTests.cs	Unit coverage for claim-based scoping and strict/non-strict behavior.
dotnet/tests/Microsoft.Agents.AI.Hosting.UnitTests/Microsoft.Agents.AI.Hosting.UnitTests.csproj	Adds a project reference to the new ASP.NET Core hosting project.
dotnet/agent-framework-dotnet.slnx	Includes the new ASP.NET Core hosting project in the solution and minor whitespace normalization.