.NET: [BREAKING] Change SerializeSession to be Async

[westey-m]

Motivation and Context

#3725

Description

• Update AIAgent.SerializeSession to be async, to match DeserializeSession and CreateSession

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

Updates the .NET AIAgent session serialization API to be asynchronous, aligning it with the existing async session creation/deserialization patterns and propagating the change through implementations, tests, and samples.

Changes:

• Replaced AIAgent.SerializeSession/SerializeSessionCore with SerializeSessionAsync/SerializeSessionCoreAsync (ValueTask-based, supports CancellationToken).
• Updated all AIAgent implementations and call sites (workflows, hosting/session store, wrappers).
• Updated unit/integration tests and samples to use the async API.

Reviewed changes

Copilot reviewed 42 out of 42 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AIAgent.cs	Introduces async session serialization API + new abstract core async method.
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentSession.cs	Updates XML docs to reference SerializeSessionAsync.
dotnet/src/Microsoft.Agents.AI.Abstractions/DelegatingAIAgent.cs	Forwards serialization via async API to inner agent.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Updates core serialization override to async ValueTask form.
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowHostAgent.cs	Updates core serialization override to async ValueTask form.
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/AIAgentHostExecutor.cs	Uses SerializeSessionAsync during checkpointing.
dotnet/src/Microsoft.Agents.AI.Hosting/Local/InMemoryAgentSessionStore.cs	Awaits SerializeSessionAsync when persisting session state.
dotnet/src/Microsoft.Agents.AI.Purview/PurviewAgent.cs	Wraps inner agent serialization via async API.
dotnet/src/Microsoft.Agents.AI.GitHub.Copilot/GitHubCopilotAgent.cs	Updates core serialization override to async ValueTask form.
dotnet/src/Microsoft.Agents.AI.CopilotStudio/CopilotStudioAgent.cs	Updates core serialization override to async ValueTask form.
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs	Updates core serialization override to async ValueTask form.
dotnet/src/Microsoft.Agents.AI.DurableTask/DurableAIAgent.cs	Updates core serialization override to async ValueTask form (Durable Task agent).
dotnet/src/Microsoft.Agents.AI.DurableTask/DurableAIAgentProxy.cs	Updates core serialization override to async ValueTask form (proxy).
dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIChatClientTests.cs	Updates test to await SerializeSessionAsync.
dotnet/tests/Microsoft.Agents.AI.UnitTests/TestAIAgent.cs	Updates test agent override signature to async core method.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentExtensionsTests.cs	Updates nested test agent override signature to async core method.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AIAgentTests.cs	Updates mock agent overrides to async core method signature.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentRunContextTests.cs	Updates nested test agent override signature to async core method.
dotnet/tests/Microsoft.Agents.AI.Hosting.AzureFunctions.UnitTests/TestAgent.cs	Updates test agent override signature to async core method.
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/AGUIEndpointRouteBuilderExtensionsTests.cs	Updates test agents to use async session serialization core override.
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/SharedStateTests.cs	Updates fake agent core serialization override + wraps returned JsonElement in ValueTask.
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/ForwardedPropertiesTests.cs	Updates fake agent core serialization override + wraps returned JsonElement in ValueTask.
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/BasicStreamingTests.cs	Updates fake agent core serialization override(s) to async signature.
dotnet/tests/Microsoft.Agents.AI.Declarative.UnitTests/AggregatorPromptAgentFactoryTests.cs	Updates nested agent core serialization override signature.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/WorkflowHostSmokeTests.cs	Updates test agent core serialization override signature.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/TestRequestAgent.cs	Updates test agent core serialization override signature.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/TestReplayAgent.cs	Updates test agent core serialization override signature.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/TestEchoAgent.cs	Wraps serialized JsonElement in ValueTask for async core override.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/Sample/06_GroupChat_Workflow.cs	Updates test agent core serialization override signature.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/RoleCheckAgent.cs	Updates test agent core serialization override signature.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/RepresentationTests.cs	Updates test agent core serialization override signature.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/InProcessExecutionTests.cs	Updates nested test agent core serialization override signature.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/AgentWorkflowBuilderTests.cs	Updates nested agent core serialization override signature.
dotnet/samples/M365Agent/AFAgentApplication.cs	Updates sample to await SerializeSessionAsync.
dotnet/samples/GettingStarted/FoundryAgents/FoundryAgents_Step06_PersistedConversations/Program.cs	Updates sample to await SerializeSessionAsync.
dotnet/samples/GettingStarted/Agents/Agent_Step20_AdditionalAIContext/Program.cs	Updates sample to await SerializeSessionAsync.
dotnet/samples/GettingStarted/Agents/Agent_Step13_BackgroundResponsesWithToolsAndPersistence/Program.cs	Updates helper to async and awaits SerializeSessionAsync.
dotnet/samples/GettingStarted/Agents/Agent_Step07_3rdPartyChatHistoryStorage/Program.cs	Updates sample to await SerializeSessionAsync.
dotnet/samples/GettingStarted/Agents/Agent_Step06_PersistedConversations/Program.cs	Updates sample to await SerializeSessionAsync.
dotnet/samples/GettingStarted/AgentWithMemory/AgentWithMemory_Step03_CustomMemory/Program.cs	Updates sample to await SerializeSessionAsync.
dotnet/samples/GettingStarted/AgentWithMemory/AgentWithMemory_Step02_MemoryUsingMem0/Program.cs	Updates sample to await SerializeSessionAsync.
dotnet/samples/GettingStarted/AgentProviders/Agent_With_CustomImplementation/Program.cs	Updates custom agent example to async core serialization override.

Comments suppressed due to low confidence (1)

dotnet/samples/GettingStarted/AgentWithMemory/AgentWithMemory_Step03_CustomMemory/Program.cs:55

• Variable name sesionElement looks like a typo; consider renaming to sessionElement (and updating its uses below) to keep the sample code clear.

JsonElement sesionElement = await agent.SerializeSessionAsync(session);

Console.WriteLine("\n>> Use deserialized session with previously created memories\n");

// Later we can deserialize the session and continue the conversation with the previous memory component state.
var deserializedSession = await agent.DeserializeSessionAsync(sesionElement);