.NET: [BREAKING] Rename ChatMessageStore to ChatHistoryProvider

[westey-m]

Motivation and Context

Currently there is confusion about ChatMessageStore. Many people assume based on the name that it is a full CRUD abstraction for storing chat history. The intention though is to just provide ChatHistory to an Agent per run and store the results of the agent.

A ChatHistoryProvider implementation may of course use a full CRUD abstraction for loading and saving messages, and we may considering providing something like that in future.

#1712

Description

• Rename ChatMessageStore to ChatHistoryProvider

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 renames ChatMessageStore to ChatHistoryProvider throughout the codebase to better reflect its purpose. The name ChatMessageStore implied it was a full CRUD abstraction for storing chat history, when the actual intention is to provide chat history to an agent per run and store the results of the agent's execution.

Changes:

• Renamed the core abstraction class from ChatMessageStore to ChatHistoryProvider
• Renamed all implementations: InMemoryChatMessageStore → InMemoryChatHistoryProvider, CosmosChatMessageStore → CosmosChatHistoryProvider, WorkflowMessageStore → WorkflowChatHistoryProvider
• Updated all properties, parameters, variables, and factory methods to use the new naming convention
• Updated all unit tests, integration tests, and samples to reflect the new naming
• Updated XML documentation and comments throughout

Reviewed changes

Copilot reviewed 43 out of 43 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
ChatHistoryProvider.cs	Core abstraction class renamed with updated documentation
InMemoryChatHistoryProvider.cs	In-memory implementation renamed with updated documentation
CosmosChatHistoryProvider.cs	Cosmos DB implementation renamed with updated documentation
WorkflowChatHistoryProvider.cs	Workflow implementation renamed
ChatClientAgent.cs	Updated to use ChatHistoryProvider throughout
ChatClientAgentThread.cs	Updated property names and documentation
ChatClientAgentOptions.cs	Updated factory property name
Extension files	Renamed extension classes and methods
Test files	Updated all test classes and methods
Sample files	Updated all sample code