.NET: Seal ChatClientAgentThread

[Copilot]

Motivation and Context

This follows the established pattern in the codebase where related classes (ChatClientAgentOptions, ChatClientAgentRunOptions, ChatClientAgentRunResponse<T>) are already sealed.

Description

Added sealed modifier to ChatClientAgentThread class declaration.

-public class ChatClientAgentThread : AgentThread
+public sealed class ChatClientAgentThread : AgentThread

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.

Original prompt

Seal ChatClientAgentThread

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[Copilot]

Pull request overview

This PR adds the sealed modifier to the ChatClientAgentThread class to align with the established pattern in the codebase where related classes (ChatClientAgentOptions, ChatClientAgentRunOptions, and ChatClientAgentRunResponse<T>) are already sealed.

Key changes:

• Modified ChatClientAgentThread from public class to public sealed class