.NET: Fixed CA1873 warning

[dmytrostruk]

Motivation and Context

Fixes CA1873 warning:

##[error]dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs(752,166): Error CA1873: Evaluation of this argument may be expensive and unnecessary if logging is disabled

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 addresses a .NET analyzer failure (CA1873) by avoiding potentially expensive log-argument evaluation when warning logging is disabled in ChatClientAgent.

Changes:

• Guard the chat history provider conflict warning log with ILogger.IsEnabled(LogLevel.Warning).
• Precompute GetLoggingAgentName() only when the warning will actually be emitted.