[BUG]: "Edit model" generates problematic code when the prompt is "Add a transition ..."

[liunelson]

Describe the bug
When you ask the agent to add new transitions, it produces code where it retrieves existing states and parameters or define new ones.

When it defines new parameters and no units are given in the prompt, it sets the units to be "". This is a bad idea since MIRA does not accept "" as a unit for parameters.

if "v" not in model.parameters: #note this is checks for paremeter's symbol
    parameter_unit = Unit(expression = sympy.Symbol(""))
    parameters = {
        "v": Parameter(name = "v", value = 1.0, units = parameter_unit, description = "Rate of transition from S to R due to vaccination")
    }
else: 
    parameters = {"v": model.parameters.get("v")}

Expected behavior
Change the agent instruction set to ensure that the generated code does not set the parameter units to be ""

    parameters = {
        "v": Parameter(name = "v", value = 1.0, description = "Rate of transition from S to R due to vaccination")
    }