Fix NPE in ParticleMathMapping for non-mass-action kinetics

[jcschaff]

Summary

• One-line guard fix in ParticleMathMapping.refreshMathDescription: NPE when a reaction's rate law cannot be reduced to mass-action form.
• transformToStochastic returns a GeneralKineticsStochasticFunction (sibling of MassActionStochasticFunction) for kinetics like Hill / Michaelis-Menten. The existing instanceof check correctly leaves maFunc null, but the next line dereferenced it unconditionally → NPE far from the real cause.
• Now throws the same descriptive MappingException (which names the offending reaction) whether maFunc is null or has both rates null.

Why now

Identified during triage of a user-support-email log corpus: this NPE is the second-largest cluster of distinct-incident failures (multiple incidents across all sampled builds, multiple distinct user models). The error users currently see is a stack-trace bottoming out at ParticleMathMapping.java:732 rather than a message naming their reaction.

Diff

// vcell-core/.../ParticleMathMapping.java:729-739
 if (stochasticFunction instanceof MassActionStochasticFunction)
 {
     maFunc = (MassActionStochasticFunction)stochasticFunction;
 }
-if ((maFunc.forwardRate() == null && maFunc.reverseRate() == null))
+// transformToStochastic falls back to GeneralKineticsStochasticFunction
+// when the rate law can't be reduced to mass-action form (e.g. Hill /
+// Michaelis-Menten kinetics). Particle math mapping requires mass-action
+// shape, so report the same error in either case rather than NPE.
+if (maFunc == null || (maFunc.forwardRate() == null && maFunc.reverseRate() == null))
 {
     throw new MappingException("Cannot generate stochastic math mapping for the reaction:" + reactionStep.getName() + ...);
 }

Test plan

• mvn compile test-compile -pl vcell-core -am
• mvn test -pl vcell-core -Dgroups="Fast" — pre-existing VCellDataTest.test_3D Poetry environmental error is unrelated to this Java change
• Manual: open a stochastic application with a non-mass-action reaction (Michaelis-Menten / Hill) and confirm the error message names the reaction instead of producing an NPE

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