PY: fix resourceType shadow warnings on family base types

[ryukzak]

Follow-up to #190.

Importing the generated package emitted Pydantic UserWarning: Field name "resourceType" ... shadows an attribute in parent for every concrete resource. The fhirpy base model registers resourceType as a class attribute from the field default in __pydantic_init_subclass__; concrete resources subclassing the abstract family types (Resource, DomainResource) then "shadow" that attribute.

• Omit the default on family/abstract types so no class attribute is registered. Their resourceType becomes a required str validated by pattern (these types are never instantiated directly).
• Guard the fhirpy class-attribute registration against PydanticUndefined so required fields are skipped.
• Update multi-package snapshots (sql-on-fhir, local-package) and regenerate the python-r4-us-core example.

Before (abstract base registered a class attribute that subclasses shadowed):

class Resource(FhirpyBaseModel):
    resourceType: str = Field(
        default='Resource',
        alias='resourceType',
        serialization_alias='resourceType',
        pattern='Resource'
    )

After (no default → no class attribute → no shadow warning):

class Resource(FhirpyBaseModel):
    resourceType: str = Field(
        alias='resourceType',
        serialization_alias='resourceType',
        pattern='Resource'
    )

Concrete resources still expose cls.resourceType for fhirpy, keep their default, and reject mismatched values.