Fix Solara render crash for CellAgent visualization

[schrodingers-cat-T10]

• [🐛 Bug fix]

Description

This PR fixes a render-time crash in the Hotelling Law Solara example
caused by accessing agent.cell.coordinate before it is fully
initialized.

When using CellAgent with OrthogonalMooreGrid, Solara may render
before all cells have stable coordinates resulting in intermittent NoneType errors.

Why this actually matters !

• Prevents confusing crashes for new Mesa users
• Aligns visualization with Mesa 3.x discrete-space semantics
• Improves robustness under Solara hot reload

Glad to contribute !

[quaquel]

Can you elaborate on the bug you are trying to fix and give an example that produces the bug?

[schrodingers-cat-T10]

@quaquel the bug popped up right way after running the application by crashing the solara site

I belive solara UI renders before updating the position of the agent in the cell

so i fixed this by adding defensive checks to skip agents whose cell or coordinate is not yet available.
This prevents render-time crashes without changing any model behavior.