.NET: Add Missing CreateSessionAsync(conversationId) to FoundryAgent

[rogerbarreto]

Motivation and Context

• Fixes .NET: [Bug]: MS Foundry Agent Multi Turn Conversations with existing Foundry Agents #5138

Users who already have a server-side conversation ID (e.g., created via \ProjectConversationsClient) cannot pass it to \FoundryAgent.CreateSessionAsync\ because the overload accepting a \conversationId\ parameter only exists on \ChatClientAgent.

Description

Adds a public \CreateSessionAsync(string conversationId, CancellationToken)\ method to \FoundryAgent\ that delegates to the inner \ChatClientAgent, allowing users to create sessions with existing server-side conversation IDs directly from the \FoundryAgent\ type.

Usage

\\csharp
// Create or obtain an existing conversation ID
ProjectConversation conversation = await conversationsClient.CreateProjectConversationAsync();

// Now directly usable on FoundryAgent
AgentSession session = await foundryAgent.CreateSessionAsync(conversation.Id);
\\

Changes

• **\FoundryAgent.cs**: Added \CreateSessionAsync(string conversationId, CancellationToken)\ that delegates to ((ChatClientAgent)this.InnerAgent).CreateSessionAsync(conversationId, cancellationToken)\
• **\FoundryAgentTests.cs**: Added 2 unit tests covering the new method and the parameterless overload

Checklist

• dotnet format passes (verified via Docker \mcr.microsoft.com/dotnet/sdk:10.0)
• All 310 existing + new tests pass on both
  et48\ and
  et10.0\

[Copilot]

Pull request overview

Adds a FoundryAgent.CreateSessionAsync(string conversationId, CancellationToken) convenience overload so callers can resume/attach to an existing server-side Foundry conversation without needing to downcast or access the inner ChatClientAgent.

Changes:

• Add FoundryAgent.CreateSessionAsync(string conversationId, CancellationToken) that delegates to the inner ChatClientAgent.
• Add unit tests asserting the returned session type and that ConversationId is set (or null for the parameterless overload).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
dotnet/src/Microsoft.Agents.AI.Foundry/FoundryAgent.cs	Adds CreateSessionAsync(conversationId) overload delegating to the inner ChatClientAgent.
dotnet/tests/Microsoft.Agents.AI.Foundry.UnitTests/FoundryAgentTests.cs	Adds unit tests for creating sessions with/without a conversation ID.