Fix: Exclude null values from list_authorized_properties response

[bokelley]

Problem

AdCP client schema validation was rejecting the list_authorized_properties response because optional fields were being set to null instead of being completely omitted from the JSON.

Error messages:

• primary_channels: Expected array, received null
• primary_countries: Expected array, received null
• portfolio_description: Expected string, received null
• advertising_policies: Expected string, received null
• last_updated: Expected string, received null
• errors: Expected array, received null

The AdCP JSON schema expects optional fields to be either:

1. Present with the correct type (array/string)
2. Completely omitted from the JSON

It does NOT accept null values for these fields.

Root Cause

When FastMCP serializes Pydantic models, it may use Python's built-in dict() function which calls __iter__ instead of the model's model_dump() method. This resulted in all fields (including those with None values) being included in the response.

Solution

1. Override __iter__() in ListAuthorizedPropertiesResponse to only yield non-None fields
2. Override dict() method to use model_dump() with exclude_none=True
3. Only pass non-None values when constructing the response object

These changes ensure optional fields are completely omitted from the JSON response rather than being present with null values, matching AdCP spec requirements.

Testing

• ✅ Unit tests pass (837 passed)
• ✅ Manual serialization testing confirms None values are excluded
• ✅ Both dict(response) and response.model_dump() now exclude None fields

Files Changed

• src/core/schema_adapters.py: Added __iter__ and dict method overrides
• src/core/tools/properties.py: Only pass non-None values to response constructor