PY: generate resourceType as str instead of Literal

[ryukzak]

Generate the resourceType field as str instead of a per-resource Literal[...]. The value is still validated on the Pydantic side via Field(pattern=...); a per-subclass Literal narrows the parent field and is redundant with the pattern.

Before:

class Observation(DomainResource):
    resourceType: Literal['Observation'] = Field(
        default='Observation',
        alias='resourceType',
        serialization_alias='resourceType',
        pattern='Observation'
    )

After:

class Observation(DomainResource):
    resourceType: str = Field(
        default='Observation',
        alias='resourceType',
        serialization_alias='resourceType',
        pattern='Observation'
    )

• Update Python writer + multi-package snapshots (cda, sql-on-fhir, local-package).
• Regenerate the python-r4-us-core example.

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Follow-up: shadow-warning fix split into #191.