[PWGHF] Fix KFParticle reconstruction in Xic0/Omegac0 candidate creator

[mtorresc15]

This PR fixes issues in the KFParticle reconstruction in candidateCreatorXic0Omegac0.cxx.

The following changes are included:

1. Fix the application of the configurable V0 and cascade mass constraints. The constrained KFParticle objects were previously redeclared inside the corresponding if blocks, shadowing the original objects. As a result, the constrained particles were not propagated to the subsequent reconstruction steps. The constrained V0, Omega, and Xi particles are now assigned to the existing KFParticle objects when the corresponding mass-constraint configurable is enabled.
2. Assign the correct charge hypothesis to the charm bachelor pion in the Omegac0 -> OmegaPi KFParticle reconstruction.
3. Correct the comment associated with the Omega mass constraint.

[github-actions]

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[fcatalan92]

Thanks @mtorresc15 !