PR: Migrate Test Agent Portrayals to AgentPortrayalStyle (Part of #2904)

This PR updates tests/test_components_matplotlib.py to use the new AgentPortrayalStyle API rather than legacy dict-based portrayals.
It reduces deprecation warnings and ensures the test suite reflects the modern visualization interface.

🔧 Issue Reference

Linked: #2904 — Migration away from deprecated dict-based agent portrayals toward AgentPortrayalStyle.

Previously, several tests defined agent_portrayal functions returning dicts like:

{"s": 10, "c": "tab:blue", ...}

These have been deprecated. The visualization system now expects an AgentPortrayalStyle instance, leading to noisy warnings and inconsistencies in the test suite.

🛠️ Implementation Details

✔ Updated Imports

from mesa.visualization.components import AgentPortrayalStyle

✔ Refactored Test Portrayal Functions

All portrayal functions in tests/test_components_matplotlib.py have been updated to return AgentPortrayalStyle instead of dicts.

✔ Key Mappings (Legacy → New API)

These mappings preserve the previous semantics—same markers, colors, sizes, and alpha values.

🧪 Testing

Executed focused test suites:

pytest tests/test_components_matplotlib.py tests/test_backends.py

✅ Result

• 21 tests passed

• Only pre-existing warnings remain

• No new errors introduced

• Tests now exercise the updated portrayal API cleanly

📌 Notes

• This PR intentionally targets only the test layer, aligning with the incremental migration plan discussed in Tracking issue: Resolve test suite warnings #2904.

• Core visualization code already supports AgentPortrayalStyle; these changes ensure tests validate the new API correctly.