fix: resolve CA1873 in GitHubCopilotAgent by using LoggerMessage source generator (microsoft#6815)

Replace the direct logger.LogWarning() call (which eagerly evaluates
string.Join()) with a [LoggerMessage]-generated extension method in
GitHubCopilotAgentLogMessages.cs.

Fixes build error:
  GitHubCopilotAgent.cs(580,13): error CA1873: Evaluation of this argument
  may be expensive and unnecessary if logging is disabled

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: alliscode

dotnet/src/Microsoft.Agents.AI.GitHub.Copilot/GitHubCopilotAgent.cs

@@ -577,9 +577,13 @@ private AgentResponseUpdate ConvertToAgentResponseUpdate(SessionEvent sessionEve
         // Warn so the developer knows the ApprovalRequiredAIFunction marker(s) will not be automatically gated.
         if (sessionConfig.Hooks?.OnPreToolUse is not null)
         {
-            logger.LogApprovalGatingSkippedDueToCustomHook(
-                approvalRequiredToolNames.Count,
-                string.Join(", ", approvalRequiredToolNames));
+            if (logger.IsEnabled(LogLevel.Warning))
+            {
+                logger.LogApprovalGatingSkippedDueToCustomHook(
+                    approvalRequiredToolNames.Count,
+                    string.Join(", ", approvalRequiredToolNames));
+            }
+
             return sessionConfig;
         }