XML serialization uses wrong naming convention for primitive extension fields

[luisfabib]

model_dump_xml() detects primitive extension fields by looking for a field named
{field_name}_ext (underscore suffix). This naming convention is non-standard.

The FHIR specification uses an underscore prefix in JSON serialisation (e.g. _birthDate
holds the extension for birthDate) and a child element with the same tag in XML. The
internal Pydantic field may follow either convention depending on how the model was generated,
but the lookup in model_dump_xml() assumes the suffix pattern in all cases.

As a result, primitive extension fields are silently missed during XML serialisation, meaning
serialised XML is missing extension data that was present in the Pydantic model.

Impact

• Round-trip JSON → model → XML → model will silently lose primitive extensions.
• Profiles that use primitive extensions (e.g. _status, _birthDate) will produce
  non-conformant XML output.