PY: Generate primitive type extension fields and restore extension/modifierExtension

[MikhailArtemyev]

Summary

• Add primitiveTypeExtension option to the Python generator — when enabled, generates _fieldName companion fields (typed as Element) for every primitive field, following the FHIR JSON representation for primitive extensions
  • Scalar primitives → Element | None (e.g. birth_date_extension: Element | None)
  • Array primitives → PyList[Element | None] | None (e.g. given_extension: PyList[Element | None] | None)
  • Auto-imports Element when the type isn't already in scope
• Restore extension and modifierExtension fields on DomainResource and BackboneElement — previously suppressed via ignoreFields in the generation config
• Add test_raw_extension.py example test (mirrors typescript-r4/raw-extension.test.ts) covering:
  • Resource-level extensions (extension, modifierExtension)
  • Element-level extensions (on HumanName, PatientContact)
  • Primitive extensions (_birthDate, _family, _given with sparse arrays)
  • Round-trip serialize → deserialize preservation

Before/After

Generated Python model — primitive fields now have _ companions:

# Before
birth_date: str | None = Field(None, alias="birthDate", ...)

# After
birth_date: str | None = Field(None, alias="birthDate", ...)
birth_date_extension: Element | None = Field(None, alias="_birthDate", ...)

Usage:

patient = Patient(
    birth_date="1990-03-15",
    birth_date_extension=Element(
        extension=[
            Extension(
                url="http://hl7.org/fhir/StructureDefinition/patient-birthTime",
                value_date_time="1990-03-15T08:22:00-05:00",
            ),
        ],
    ),
)

[ryukzak]

• Let's try to use https://pypi.org/project/pytest-snapshot/ to reduce the amount of test libs.
• Let's add to the test how we can get extension data from resources. Looks like res._gender will not work.