.NET: fix(agui): create session in MapAGUI endpoint so middleware receives …

[kallebelins]

…non-null session (#3823)

MapAGUI was calling RunStreamingAsync without passing the session parameter, causing it to always be null for all middleware decorators registered via AIAgentBuilder.Use().

Changes:

• AGUIEndpointRouteBuilderExtensions.cs: create an AgentSession before invoking RunStreamingAsync and pass it through, so middleware receives a non-null session.
• Add integration tests (SessionMiddlewareTests) covering:
  • Session propagation to streaming middleware
  • Session available via CurrentRunContext
  • Session propagation to shared Use() overload
  • Session.StateBag is functional

Fixes #3823

Motivation and Context

When an agent is hosted via the AG-UI endpoint (MapAGUI) and middleware is registered through AIAgentBuilder.Use(), the session parameter passed to middleware delegates is always null. This prevents middleware from accessing session state, reading CurrentRunContext.Session, or using Session.StateBag for cross-cutting concerns such as logging, telemetry, or state management.

This change fixes the root cause by creating an AgentSession in the MapAGUI endpoint before invoking RunStreamingAsync, ensuring the session is properly propagated through the entire middleware pipeline.

Fixes #3823

Description

The fix modifies AGUIEndpointRouteBuilderExtensions.cs to create a session instance via agent.CreateSessionAsync() before calling RunStreamingAsync, and passes it as the session parameter. Previously, the call omitted the session argument entirely, defaulting it to null.

A new integration test file SessionMiddlewareTests.cs is added to Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests with 4 regression tests that exercise the full round-trip (AGUIChatClient → HTTP → MapAGUI → middleware → agent) and validate:

1. AGUIMiddleware_WithWorkflow_SessionIsNotNullAsync — session is non-null in streaming middleware via Use(runStreamingFunc:).
2. AGUIMiddleware_CurrentRunContext_SessionIsNotNullAsync — AIAgent.CurrentRunContext.Session is non-null during AGUI-initiated runs.
3. AGUIMiddleware_SharedUseOverload_SessionIsNotNullAsync — session is non-null in the shared pre/post-processing Use() overload.
4. AGUIMiddleware_SessionStateBag_IsAccessibleAsync — Session.StateBag is functional (set/get values) when invoked via AGUI endpoint.

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.

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[Copilot]

Pull request overview

This PR fixes a bug where the session parameter was always null in middleware when agents are hosted via the AGUI endpoint (MapAGUI). The root cause was that RunStreamingAsync was called without passing the session parameter, causing it to default to null for all middleware decorators registered via AIAgentBuilder.Use().

Changes:

• Modified AGUIEndpointRouteBuilderExtensions.cs to create an AgentSession before invoking RunStreamingAsync and pass it through the middleware pipeline
• Added comprehensive integration test suite SessionMiddlewareTests.cs with 4 tests verifying session propagation through different middleware scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs	Creates session via CreateSessionAsync before calling RunStreamingAsync, ensuring middleware receives non-null session
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/SessionMiddlewareTests.cs	Adds 4 integration tests validating session propagation to streaming middleware, CurrentRunContext, shared Use() overload, and StateBag functionality

[kallebelins]

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