Do not encode a label for a signal without an MC particle

This fixes a problem in MCCompLabel and adds a unit test.

- A hit whose track was pruned carries trackID -1, and the constructor encoded
  that into the 31 bit track field.
- Decoding then gave trackID 2147483647 with isValid() true, so the label
  looked like a correctly identified particle.
- The constructor now leaves the label unset for a negative trackID, which is
  the state a decoder already handles.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: sawenzel

DataFormats/simulation/include/SimulationDataFormat/MCCompLabel.h

@@ -52,7 +52,14 @@ class MCCompLabel
   // mask for all used fields
   static constexpr uint64_t maskFull = (ul0x1 << (nbitsTrackID + nbitsEvID + nbitsSrcID)) - 1;
 
-  MCCompLabel(int trackID, int evID, int srcID, bool fake = false) { set(trackID, evID, srcID, fake); }
+  MCCompLabel(int trackID, int evID, int srcID, bool fake = false)
+  {
+    // a negative trackID means no MC particle is attached to this signal;
+    // the label stays unset rather than encoding a track that does not exist
+    if (trackID >= 0) {
+      set(trackID, evID, srcID, fake);
+    }
+  }
   MCCompLabel(bool noise = false)
   {
     if (noise) {

DataFormats/simulation/test/testMCCompLabel.cxx

@@ -53,3 +53,10 @@ BOOST_AUTO_TEST_CASE(MCCompLabel_test)
   MCCompLabel dummy;
   BOOST_CHECK(dummy.isEmpty() && !dummy.isNoise() && dummy.isFake() && !dummy.isValid());
 }
+
+// A hit whose track was pruned carries trackID -1 and has no MC particle
+BOOST_AUTO_TEST_CASE(MCCompLabel_no_particle_test)
+{
+  MCCompLabel unmapped(-1, 200, 10);
+  BOOST_CHECK(!unmapped.isValid());
+}