Pre-PR Checklist

• This PR is a bug fix, not a new feature or enhancement.

Summary

Fixes #3184 — agent.meta_agent was set non-deterministically when an agent belongs to multiple meta-agents.

Bug / Issue

Three locations in meta_agent.py blindly set agent.meta_agent = self without checking existing memberships:

• MetaAgent.__init__ (line 315) — overwrites any existing assignment
• add_constituting_agents (line 391) — same issue
• create_meta_agent path 1 (line 246) — iterates over a.meta_agents (set()) which has non-deterministic order

Different runs produce different agent.meta_agent pointers for the same model, breaking reproducibility.

Implementation

All three locations now pick the meta-agent with the lowest unique_id:

agent.meta_agent = sorted(
    agent.meta_agents, key=lambda x: x.unique_id or 0
)[0]

Consistent with the existing pattern in remove_constituting_agents (lines 404-409).

Testing

All 20 meta-agent tests pass. No new tests needed — fix makes existing behavior deterministic without changing semantics.

Additional Notes

None.

GSoC contributor checklist

Context & motivation

I found this bug while reading through #3184 and tracing how overlapping membership works in meta-agents. agent.meta_agent was just being set to self everywhere without checking if the agent already belonged to another meta-agent. The pointer ends up depending on creation order, which is not predictable. Reproducibility matters for any simulation framework — same inputs should give same outputs.

What I learned

The interesting part was that remove_constituting_agents already had the correct deterministic pattern (sorting by unique_id) but __init__ and add_constituting_agents didn't follow it. I also learned that iterating over a Python set() is non-deterministic across runs, so even the create_meta_agent path needed sorting. The fix was about consistency — applying the same pattern that already existed in one place to the others that were missing it.

Learning repo

My learning repo: https://github.com/codebyNJ/GSoC-learning-space

Readiness checks

• This PR addresses an agreed-upon problem (linked issue additional bugs in meta_agents #3184)
• I have read the contributing guide and deprecation policy
• I have performed a self-review
• Tests pass locally (pytest --cov=mesa tests/)
• Code is formatted (ruff check . --fix)