Add default_value and value_type to Variable class

[anth-volk]

Summary

Add default_value and value_type fields to the Variable class in policyengine.py to expose these properties from policyengine-core.

Motivation

The policyengine-api-v2-alpha and policyengine-app-v2 need access to variable default values to properly initialize variables when users add them to household builders. Currently, the Variable class in policyengine.py only exposes:

• id, name, entity, description, data_type, possible_values

But policyengine-core's Variable class also has:

• default_value - the default value for the variable (e.g., 0 for floats, False for bools, "SINGLE" for enums)
• value_type - the Python type of the variable (float, bool, int, str, Enum, datetime.date)

Without these fields, downstream tools must load the entire country-specific CountryTaxBenefitSystem just to access default values, which defeats the purpose of policyengine.py as a consistent abstraction layer.

Changes Required

1. src/policyengine/core/variable.py: Add fields to Variable class:

   default_value: Any = None
   value_type: type | None = None

2. src/policyengine/tax_benefit_models/us/model.py and uk/model.py: Extract and serialize values:

   default_val = var_obj.default_value
   if var_obj.value_type is Enum:
       default_val = default_val.name
   elif var_obj.value_type is datetime.date:
       default_val = default_val.isoformat()
   
   variable = Variable(
       # ... existing fields ...
       default_value=default_val,
       value_type=var_obj.value_type,
   )

Supported value_types (from policyengine-core config)

value_type	default
bool	False
int	0
float	0
str	""
Enum	enum member name (string)
datetime.date	"1970-01-01" (ISO format)

Branch

Implementation available on branch: feat/add-variable-default-value