SolaraViz: Improve Portrayal API

However, I'm wondering if we should consider improving this API in a follow-up. The current design feels a bit awkward:
- Simple cases require writing a function wrapper that always returns the same object
- The function --> class pattern isn't the most intuitive

Right now users must write:
```python
def propertylayer_portrayal(layer):
return PropertyLayerStyle(colormap="viridis", colorbar=True)
```

Where the function wrapper adds no value when the style is uniform across all layers.

What if we allowed the visualization to accept either:
1. A `PropertyLayerStyle` instance directly (for simple uniform styling)
2. A callable function (for conditional/per-layer styling)

Similar to how Python's `sorted()` accepts both `key=str.lower` and `key=lambda x: complex_logic(x)`.

_Originally posted by @EwoutH in https://github.com/projectmesa/mesa/pull/2912#discussion_r2589989850_

The issue names no files or tests; start by reviewing the portrayal API discussion in PR #2912 and locating the current SolaraViz portrayal entry point. Done means uniform styling can use a PropertyLayerStyle instance directly while conditional styling continues to support a callable.

Written by the indexing model from the issue text.