Motivation and Context

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

Description

Addedsealed modifier toChatClientAgentThread class declaration.

-public class ChatClientAgentThread: AgentThread+publicsealedclassChatClientAgentThread:AgentThread

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows theContribution 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.

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