.NET: [BREAKING] Move AgentSession.Serialize to AIAgent

[westey (westey-m)]

Motivation and Context

#3646

Description

• Move AgentSession.Serialize to AIAgent

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

This PR refactors the serialization API by moving the Serialize method from AgentSession to AIAgent as SerializeSession. This change makes the API more consistent by centralizing session lifecycle management (creation, serialization, and deserialization) on the AIAgent class rather than splitting it between agent and session.

Changes:

• Removed public virtual Serialize method from AgentSession base class
• Added public abstract SerializeSession method to AIAgent that takes a session parameter
• Updated all concrete agent implementations to implement SerializeSession with type checking
• Changed session-level Serialize methods to internal or protected internal virtual visibility
• Updated all usages in samples, tests, and production code to call agent.SerializeSession(session) instead of session.Serialize()

Reviewed changes

Copilot reviewed 51 out of 51 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentSession.cs	Removed public Serialize method and updated documentation
dotnet/src/Microsoft.Agents.AI.Abstractions/AIAgent.cs	Added public abstract SerializeSession method
dotnet/src/Microsoft.Agents.AI.Abstractions/DelegatingAIAgent.cs	Added delegation of SerializeSession to inner agent
dotnet/src/Microsoft.Agents.AI.Abstractions/InMemoryAgentSession.cs	Changed Serialize to protected internal virtual
dotnet/src/Microsoft.Agents.AI.Abstractions/ServiceIdAgentSession.cs	Changed Serialize to protected internal virtual
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Implemented SerializeSession with type validation
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentSession.cs	Changed Serialize to internal
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowHostAgent.cs	Implemented SerializeSession with type validation
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowSession.cs	Changed Serialize to internal
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/AIAgentHostExecutor.cs	Updated to use agent.SerializeSession instead of session.Serialize
dotnet/src/Microsoft.Agents.AI.Purview/PurviewAgent.cs	Added delegation of SerializeSession to inner agent
dotnet/src/Microsoft.Agents.AI.Hosting/Local/InMemoryAgentSessionStore.cs	Updated to use agent.SerializeSession
dotnet/src/Microsoft.Agents.AI.GitHub.Copilot/GitHubCopilotAgent.cs	Implemented SerializeSession with type validation
dotnet/src/Microsoft.Agents.AI.GitHub.Copilot/GitHubCopilotAgentSession.cs	Changed Serialize to internal
dotnet/src/Microsoft.Agents.AI.DurableTask/DurableAIAgent.cs	Implemented SerializeSession with type validation
dotnet/src/Microsoft.Agents.AI.DurableTask/DurableAIAgentProxy.cs	Implemented SerializeSession with type validation
dotnet/src/Microsoft.Agents.AI.DurableTask/DurableAgentSession.cs	Changed Serialize to internal
dotnet/src/Microsoft.Agents.AI.CopilotStudio/CopilotStudioAgent.cs	Implemented SerializeSession with type validation
dotnet/src/Microsoft.Agents.AI.CopilotStudio/CopilotStudioAgentSession.cs	Added internal new Serialize method
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs	Implemented SerializeSession with type validation
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgentSession.cs	Changed Serialize to internal
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentSessionTests.cs	Removed obsolete test for old Serialize method
dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIChatClientTests.cs	Updated test to use agent.SerializeSession
dotnet/tests/Microsoft.Agents.AI.DurableTask.UnitTests/DurableAgentSessionTests.cs	Test calls internal Serialize directly (valid for internal testing)
dotnet/tests/**/*.cs (multiple test files)	Added NotImplementedException stubs for SerializeSession in test agents
dotnet/samples/**/*.cs (multiple sample files)	Updated to use agent.SerializeSession pattern