[client] Use ctx.Err() instead of gRPC codes.Canceled to detect shutdown (#6019)

Detecting shutdown by inspecting the gRPC status code conflates a local
context cancellation with a server- or proxy-sent codes.Canceled. When
the latter occurs (e.g. an intermediary proxy resets the stream), the
retry loop silently terminates and the client never reconnects.

Switch to ctx.Err() in the signal Receive loop and management Sync/Job
handlers, and stop matching gRPC Canceled/DeadlineExceeded in the flow
client's isContextDone helper. With this change, a server-sent Canceled
is treated as a transient error and the backoff retry loop continues.

shared/signal/client/grpc.go

@@ -167,7 +167,7 @@ func (c *GrpcClient) Receive(ctx context.Context, msgHandler func(msg *proto.Mes
 		// start receiving messages from the Signal stream (from other peers through signal)
 		err = c.receive(stream)
 		if err != nil {
-			if s, ok := status.FromError(err); ok && s.Code() == codes.Canceled {
+			if ctx.Err() != nil {
 				log.Debugf("signal connection context has been canceled, this usually indicates shutdown")
 				return nil
 			}