In the old visualisation you could add values like "Shape": "circle" and "Filled": "true" to the agent_portrayal dict.

With the new Solara based visualisation, that's not yet possible. It should be implemented in mesa/visualization/components. Somewhere around here:
https://github.com/projectmesa/mesa/blob/74e2df6fdfc2d5172f84eb96469b277b35bfd795/mesa/visualization/components/altair.py#L50-L55
and maybe also in:
https://github.com/projectmesa/mesa/blob/74e2df6fdfc2d5172f84eb96469b277b35bfd795/mesa/visualization/components/matplotlib.py#L40-L47