.NET: Chathistory memory provider add

[westey-m]

Motivation and Context

#1680

Description

• Add an AIContextProvider that stores chat history in a vector store and can find related messages for context later.

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 new vector data memory provider for the Microsoft Agents framework. The implementation enables storing and retrieving chat history using vector stores with semantic similarity search capabilities.

Key changes:

• New ChatHistoryMemoryProvider class that implements AIContextProvider to store chat messages in a vector store and retrieve relevant context
• Support for scoping chat history by application, agent, thread, and user through ChatHistoryMemoryProviderScope
• Configurable search behavior (automatic vs. on-demand function calling) via ChatHistoryMemoryProviderOptions

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.VectorDataMemory/ChatHistoryMemoryProvider.cs	Core implementation of the chat history memory provider with vector store integration
dotnet/src/Microsoft.Agents.AI.VectorDataMemory/ChatHistoryMemoryProviderScope.cs	Defines scoping parameters for filtering chat history
dotnet/src/Microsoft.Agents.AI.VectorDataMemory/ChatHistoryMemoryProviderOptions.cs	Configuration options for search behavior and context formatting
dotnet/src/Microsoft.Agents.AI.VectorDataMemory/VectorDataMemoryJsonUtilities.cs	JSON serialization utilities with AOT/trimming support
dotnet/src/Microsoft.Agents.AI.VectorDataMemory/Microsoft.Agents.AI.VectorDataMemory.csproj	Project file for the new library
dotnet/tests/Microsoft.Agents.AI.VectorDataMemory.UnitTests/ChatHistoryMemoryProviderTests.cs	Comprehensive unit tests covering various scenarios
dotnet/tests/Microsoft.Agents.AI.VectorDataMemory.UnitTests/Microsoft.Agents.AI.VectorDataMemory.UnitTests.csproj	Test project configuration
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentThread.cs	Updated to handle undefined JSON values in serialization
dotnet/agent-framework-dotnet.slnx	Added new projects to solution
dotnet/Directory.Packages.props	Added VectorData.Abstractions package dependency