.NET: Add Unit test for declarative object model AddConversationMessageExecutor

[peibekwe]

Motivation and Context

Unit test for declarative object model AddConversationMessageExecutor

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 comprehensive unit tests for the AddConversationMessageExecutor class, which is part of the declarative object model for workflows. The tests verify the executor's ability to add conversation messages with different roles and validate the message creation process.

Key Changes

• Added AddConversationMessageExecutorTest with tests for user and agent roles
• Added test for message creation without explicit role assignment
• Updated MockAgentProvider to support the CreateMessageAsync operation

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
AddConversationMessageExecutorTest.cs	New test file with comprehensive unit tests for AddConversationMessageExecutor, including Theory-based tests for different message roles and Fact-based test for default role behavior
MockAgentProvider.cs	Extended mock provider to include setup for CreateMessageAsync method, returning the first test message for all invocations