.NET: Add AGUI session persistence

[Kazunari001]

Motivation and Context

Fix #4869

Description

• add an in-memory AGUI session store and register it in DI
• persist agent sessions by thread id during streaming runs
• add integration tests for conversation and session restoration
• add unit tests for cache key isolation and session expiration

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.

[Copilot]

Pull request overview

Adds server-side, in-memory session persistence for the ASP.NET Core AG-UI hosting endpoint to ensure conversation/session continuity across requests using thread IDs (fixing #4869).

Changes:

• Introduces an in-memory AGUIInMemorySessionStore and registers it in DI via AddAGUI().
• Updates MapAGUI streaming execution to load/create a session by ThreadId and persist it after streaming completes.
• Adds unit + integration tests validating cache-key isolation, expiration, and cross-request/session restoration behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIInMemorySessionStore.cs	Adds an in-memory MemoryCache-backed store for AgentSession keyed by agent/thread.
dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/ServiceCollectionExtensions.cs	Registers the in-memory session store in DI as a singleton when calling AddAGUI().
dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs	Loads/creates sessions by ThreadId and persists them during streaming runs.
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/AGUIInMemorySessionStoreTests.cs	Unit tests for reuse, key isolation, and expiration behavior.
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/BasicStreamingTests.cs	Integration tests validating conversation/session restoration across calls.

[Kazunari001]

@copilot apply changes based on the comments in this thread

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.