fix: AdCP responses now exclude None values in JSON serialization

[bokelley]

Problem

The AdCP client (adcp/client@2.5.1) validates responses against JSON schemas that don't allow null for optional fields. When calling list_authorized_properties with minimal configuration, the response included null values for optional fields like primary_channels, primary_countries, etc., causing validation errors:

Schema validation failed: primary_channels: Expected array, received null

Root Cause

• AdCPBaseModel.model_dump() correctly excluded None values (exclude_none=True by default)
• However, model_dump_json() did not inherit this behavior (Pydantic requires explicit configuration for JSON serialization)
• FastMCP and A2A protocol layers may use model_dump_json() for serialization
• Result: JSON output contained "field": null instead of omitting the field entirely

Solution

Override model_dump_json() in AdCPBaseModel to default exclude_none=True, matching the behavior of model_dump(). This ensures:

• Optional fields with None values are omitted from JSON output
• Matches AdCP spec requirement: optional fields should be absent, not null
• Consistent serialization behavior across both dict and JSON outputs

Testing

• Added tests/unit/test_adcp_json_serialization.py with 4 comprehensive test cases
• All 837 existing unit tests still pass
• Verified list_authorized_properties, get_products, and list_creative_formats all correctly exclude None values

Example

Before:

{
  "publisher_domains": ["example.com"],
  "primary_channels": null,
  "primary_countries": null,
  "portfolio_description": null,
  "advertising_policies": null
}

After:

{
  "publisher_domains": ["example.com"]
}

Impact

• Fixes client-side validation errors for all AdCP response types
• No breaking changes (only removes fields that were null anyway)
• All existing tests pass without modification

Fixes validation errors in adcp/client@2.5.1