fix: enforce Extension.url required validation per FHIR spec (Fixes #205)

[rtmalikian]

Fixes #205

Problem

FHIR Extension.url has cardinality 1..1 in the spec — it is always required. The model had "element_required": True in json_schema_extra metadata, but Pydantic v2 does not enforce custom metadata during validation. The field also had default=None, making it effectively optional.

This allowed invalid data like {"valueCode": "F"} (no url) to pass validation:

from fhir.resources.R4B.patient import Patient
Patient.model_validate({
    "resourceType": "Patient",
    "extension": [{"valueCode": "F"}],
})  # Succeeds — should fail

Solution

Added a model_validator(mode='after') to Extension in all three FHIR versions (R4B, R5, STU3) that raises ValidationError when an Extension has value[x] fields or nested extensions but no url. Empty extensions (no url, no value) are tolerated for backward compatibility.

Verification

tests/test_extension_url_required.py::TestExtensionUrlRequired::test_r4b_extension_with_value_requires_url PASSED
tests/test_extension_url_required.py::TestExtensionUrlRequired::test_r5_extension_with_value_requires_url PASSED
tests/test_extension_url_required.py::TestExtensionUrlRequired::test_stu3_extension_with_value_requires_url PASSED
tests/test_extension_url_required.py::TestExtensionUrlRequired::test_r4b_valid_extension_with_url PASSED
tests/test_extension_url_required.py::TestExtensionUrlRequired::test_r4b_extension_url_only PASSED
tests/test_extension_url_required.py::TestExtensionUrlRequired::test_r4b_extension_nested_requires_url PASSED
tests/test_extension_url_required.py::TestExtensionUrlRequired::test_r4b_value_string_requires_url PASSED
tests/test_summary_mode_serialization.py::test_summary_for_model_serialization PASSED
tests/test_summary_mode_serialization.py::test_summary_for_json_serialization PASSED
tests/test_summary_mode_serialization.py::test_summary_for_xml_serialization PASSED
tests/test_summary_mode_serialization.py::test_summary_for_yaml_serialization PASSED
tests/test_xml_validate_and_dump.py::test_xml_node_patient_resource PASSED
tests/test_xml_validate_and_dump.py::test_xml_node_observation_resource PASSED
tests/test_xml_validate_and_dump.py::test_element_to_node PASSED
tests/test_xml_validate_and_dump.py::test_model_obj_xml_file PASSED
tests/test_yaml_validate_and_dump.py::test_yaml_patient_resource PASSED

16 passed in 0.54s

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About the Author: Raphael Malikian — Clinical AI Solutions Architect. I specialise in building and fixing AI/ML systems for healthcare, including vector databases, RAG pipelines, and clinical NLP. If you need help with your project or think I can add value to your organisation, feel free to reach out — I'd love to connect.

📧 rtmalikian@gmail.com
🔗 GitHub: https://github.com/rtmalikian
🔗 LinkedIn: http://www.linkedin.com/in/raphael-t-malikian-mbbs-bsc-hons-71075436a

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Disclosure: This code was developed with assistance from mimo-2.5-pro (Xiaomi) via Hermes Agent (Nous Research). All changes were reviewed, tested against the actual codebase, and verified for correctness.