.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
+publicsealedclassChatClientAgentThread: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

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[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