PY: make python-us-core example mypy --strict clean

[ryukzak]

Follow-up from #180. That PR widened the us-core mypy scope to the whole example dir but kept the relaxed `mypy.ini`. Moving to `--strict` is a larger, separate effort.

Running `mypy --strict --config-file mypy.ini .` on `examples/python-us-core` currently reports 250 errors.

Generated code — 171 (needs generator changes)

• `no-any-return` (87): getters declared with a concrete return type but `return item` / `model_dump()` yields `Any` — needs casts or typed helpers.
• `type-arg` (77): bare generics in output — `dict` (flat slice form) and FHIR base types without their `[str]` param (`Coding`, `CodeableConcept`, …) — generator must emit type args.
• `unused-ignore` (7): stale ignores in generated code.

Hand-written tests/demo — 79

• `no-untyped-def` (60): test/demo functions need return annotations.
• `unused-ignore` (10), `attr-defined` (5, mostly `init` not re-exporting profile classes), `comparison-overlap` (4).

Notes

• The other two python examples (`python-r4`, `python-fhirpy`) already run `mypy --strict .`; this would bring us-core in line.
• The big two (`type-arg`, `no-any-return`) are the bulk and require generator work in `src/api/writer-generator/python/`.
• Possible incremental step before full strict: enable `warn_unused_ignores` and `disallow_untyped_defs` for test files.