.NET: Add helpers to more easily access in-memory ChatHistory and make ChatHistoryProvider management more configurable.

[westey-m]

Motivation and Context

#4140

Description

• Add helpers to more easily access in-memory ChatHistory.
• Make ChatHistoryProvider management on ChatClientAgent more configurable.

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 adds helper methods to access in-memory chat history from agent sessions and introduces configurable ChatHistoryProvider management options for ChatClientAgent, addressing issue #4140 where users could no longer easily access session messages in RC1.

Changes:

• Added TryGetInMemoryChatHistory and SetInMemoryChatHistory extension methods on AgentSession for easier access to in-memory chat history
• Introduced three new configuration options on ChatClientAgentOptions to control behavior when ChatHistoryProvider conflicts with server-side chat history: ThrowOnChatHistoryProviderConflict, WarnOnChatHistoryProviderConflict, and ClearOnChatHistoryProviderConflict
• Modified ChatClientAgent constructor to eagerly initialize ChatHistoryProvider with InMemoryChatHistoryProvider when none is provided, and updated conflict detection to distinguish between user-provided and default providers

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentSessionExtensions.cs	New extension methods providing convenient access to in-memory chat history state
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentOptions.cs	Added three boolean properties to control ChatHistoryProvider conflict handling behavior with appropriate defaults
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Changed ChatHistoryProvider initialization from lazy to eager, updated conflict detection logic to check options instead of instance property
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentLogMessages.cs	Added warning log message for ChatHistoryProvider conflicts
dotnet/samples/GettingStarted/Agents/Agent_Step16_ChatReduction/Program.cs	Updated to use new TryGetInMemoryChatHistory extension method instead of accessing provider directly
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentSessionExtensionsTests.cs	Comprehensive tests for new extension methods covering all scenarios
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_ChatHistoryManagementTests.cs	New tests validating all conflict resolution behaviors
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentOptionsTests.cs	Updated tests to verify new properties and their defaults