Resolved bug in the selection of particle's default z value when it is not provided by the user

[wyatt-fluidnumerics]

Description

Previously the logic to initialize the depth level of particles when no depth was provided did not work as intended, and lead to the initialization of particles out of bounds. This fixes that by selected the depth level from the fields that is closest to zero.

Checklist

• Closes ParticleSet default initial z not set correctly (does not respect direction of depth axis) #2772
• This PR targets the correct branch (main for normal development, v3-support for v3 support)

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
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  • Describe how you used it (e.g., by pasting your prompt): Once I had written the new logic, I had Claude Code generate a few test cases.