.NET: Introduce Core implementation methods for session methods on AIAgent

[westey-m]

Motivation and Context

For all run methods on AIAgent we currently have public and protected abstract Core methods for doing the main implementation.
This allows us to do some common work in AIAgent that should apply to all agents.
Extending this pattern to the session methods as well.

Description

• Add Core methods for CreateSession, SerializeSession and DeserializeSession

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 introduces a consistent pattern for session management methods in the AIAgent base class by adding "Core" implementation methods. The changes extend an existing architectural pattern where public methods delegate to protected abstract Core methods, allowing AIAgent to perform common work before delegating to specific implementations.

Changes:

• Added public wrapper methods (CreateSessionAsync, SerializeSession, DeserializeSessionAsync) and corresponding protected abstract Core methods in AIAgent base class
• Updated all agent implementations to override the new protected Core methods instead of public abstract methods
• Updated test mocks to properly mock the protected Core methods

Reviewed changes

Copilot reviewed 32 out of 32 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AIAgent.cs	Added public wrapper methods and protected abstract Core methods for session management
dotnet/src/Microsoft.Agents.AI.Abstractions/DelegatingAIAgent.cs	Updated to override Core methods and delegate to inner agent
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Changed session methods from public override to protected override Core methods
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowHostAgent.cs	Changed session methods from public override to protected override Core methods
dotnet/src/Microsoft.Agents.AI.Purview/PurviewAgent.cs	Changed session methods from public override to protected override Core methods
dotnet/src/Microsoft.Agents.AI.GitHub.Copilot/GitHubCopilotAgent.cs	Changed session methods from public override to protected override Core methods
dotnet/src/Microsoft.Agents.AI.DurableTask/DurableAIAgentProxy.cs	Changed session methods from public override to protected override Core methods
dotnet/src/Microsoft.Agents.AI.DurableTask/DurableAIAgent.cs	Changed session methods from public override to protected override Core methods
dotnet/src/Microsoft.Agents.AI.CopilotStudio/CopilotStudioAgent.cs	Changed session methods from public override to protected override Core methods
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs	Changed session methods from public override to protected override Core methods
dotnet/samples/GettingStarted/AgentProviders/Agent_With_CustomImplementation/Program.cs	Updated sample code to override Core methods
dotnet/tests/* (multiple test files)	Updated all test agent implementations to override Core methods and fixed mock setups