.NET: Allow overriding the ChatMessageStore to be used per agent run.

[westey-m]

Motivation and Context

#3104

Description

• Allow overriding the ChatMessageStore to be used per agent run via AdditionalProperties
• Moved all ChatMessageStore and ConversationId tests into a separate test file to reduce the side of the main ChatClientAgentTests file.

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 pull request adds functionality to allow overriding the ChatMessageStore on a per-agent-run basis via AdditionalProperties. The PR also reorganizes tests by moving ChatMessageStore and ConversationId related tests into a separate file.

Changes:

• Added AdditionalPropertiesExtensions with type-based key storage/retrieval for AdditionalPropertiesDictionary
• Implemented ResolveChatMessageStore to support ChatMessageStore overrides via AdditionalProperties
• Made ChatMessageStoreMessages nullable in InvokedContext to handle first-run scenarios
• Reorganized tests by moving chat history management tests to a dedicated file

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AdditionalPropertiesExtensions.cs	New extension methods for type-based storage/retrieval in AdditionalPropertiesDictionary
dotnet/src/Microsoft.Agents.AI.Abstractions/ChatMessageStore.cs	Made ChatMessageStoreMessages parameter nullable in InvokedContext constructor
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Added ResolveChatMessageStore method and updated all notification call sites to pass chatOptions
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AdditionalPropertiesExtensionsTests.cs	Comprehensive tests for AdditionalPropertiesExtensions
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_ChatHistoryManagementTests.cs	New test file consolidating ChatMessageStore and ConversationId tests, including override test
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs	Removed tests that were moved to ChatHistoryManagementTests
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_DeserializeThreadTests.cs	Namespace changed for consistency
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_GetNewThreadTests.cs	Namespace changed for consistency
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentContinuationTokenTests.cs	Namespace changed for consistency