fix(mesh-phase5): telemetry service — graceful client-disconnect + null-safe required-string mapping

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Server/ZB.MOM.WW.OtOpcUa.Host/Grpc/TelemetryStreamGrpcService.cs

@@ -103,6 +103,15 @@ public sealed class TelemetryStreamGrpcService : GeneratedServiceBase
                 // Normal termination: the client disconnected (context token) or the max-lifetime cap
                 // fired (linked token). Neither is an error — swallow and let the stream close cleanly.
             }
+            catch (Exception ex) when (ex is IOException or RpcException)
+            {
+                // Routine mid-stream disconnect: central's TelemetryStreamClient reconnects on ANY
+                // non-Cancelled error, so a broken pipe / connection-reset here is expected churn, not a
+                // fault. Swallow at Debug so grpc-dotnet doesn't log it at Error on every reconnect.
+                // (A genuinely unexpected exception still escapes to surface as an error.)
+                _log.LogDebug(
+                    ex, "Telemetry client disconnected mid-stream (correlationId={CorrelationId})", correlationId);
+            }
 
             _log.LogDebug("Telemetry stream closed (correlationId={CorrelationId})", correlationId);
         }