test(logger): add debug log test to verify debug level works

Added test debug log immediately after logger initialization.
If debug logging is working, we should see this message.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/mpc-system/services/session-coordinator/cmd/server/main.go

@@ -59,6 +59,8 @@ func main() {
 	}
 	defer logger.Sync()
 
+	// Test debug logging
+	logger.Debug("TEST: This is a DEBUG level log - if you see this, debug logging works!")
 	logger.Info("Starting Session Coordinator Service",
 		zap.String("environment", cfg.Server.Environment),
 		zap.Int("grpc_port", cfg.Server.GRPCPort),