fix(mqtt): SparkplugCodec's never-throw guard must span the projection too

The try/catch stopped at Payload.Parser.ParseFrom, leaving the metric-projection
loop outside it — so the "never throws for any input" contract had a hole in
exactly the part that walks an attacker-shaped object graph. Widened to cover
parse AND projection.

Self-review finding; no test reddened, because a projection escape needs a
generated-code shape the vectors do not produce. That is the point: the guard is
the contract, not the observed behaviour of today's inputs.

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

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt/Sparkplug/SparkplugCodec.cs

@@ -74,19 +74,9 @@ public static class SparkplugCodec
             return false;
         }
 
-        Payload proto;
         try
         {
-            proto = Payload.Parser.ParseFrom(wire);
+            var proto = Payload.Parser.ParseFrom(wire);
-        }
-        catch (Exception)
-        {
-            // Deliberately broad. InvalidProtocolBufferException covers malformed, truncated and
-            // over-nested input, but this is the dispatcher-thread boundary: the contract is a
-            // verdict for EVERY input, and narrowing the catch would trade that guarantee for a
-            // taxonomy nobody downstream can act on.
-            return false;
-        }
 
             var metrics = proto.Metrics.Count == 0
                 ? []
@@ -105,6 +95,18 @@ public static class SparkplugCodec
 
             return true;
         }
+        catch (Exception)
+        {
+            // Deliberately broad, and deliberately spanning the projection as well as the parse.
+            // InvalidProtocolBufferException covers malformed, truncated and over-nested input, but
+            // this is the dispatcher-thread boundary and the projection walks an attacker-shaped
+            // object graph: the contract is a verdict for EVERY input, so a guard that stopped at
+            // the parse would be a contract with a hole in it. Narrowing the catch would trade the
+            // guarantee for a taxonomy nobody downstream can act on.
+            payload = SparkplugPayload.Invalid;
+            return false;
+        }
+    }
 
     /// <summary>
     ///     Decodes Sparkplug-B wire bytes, returning <see cref="SparkplugPayload.Invalid"/> when they