py: generics

[MikhailArtemyev]

Generator changes

• Python generator reads schema.generic?.params from the IR and
  emits TypeVar declarations and Generic[T1, T2] superclasses for
  Bundle, BundleEntry, BundleEntryResponse, and DomainResource
• Resource-typed fields (BundleEntry.resource,
  DomainResource.contained) become TypeVars instead of
  ResourceFamily
• New static asset resource_preprocessor.py — copied per FHIR
  package when any schema has resource generic params
• Each generic class gets a model_validator(mode='before') calling
  preprocess_resource_fields, restoring polymorphic dispatch by
  resourceType for bare Bundle.from_json() usage
• Removed resource_family_validator.py asset and all
  resource_families.py generation — ResourceFamily types fully
  replaced

Before / after

Resource field type:

# Before
entry: ResourceFamily | None
                         
# After
entry: T1 | None  # T1 = TypeVar('T1', bound=Resource,
default=Resource)

Bare Bundle.from_json() now works again:
bundle = Bundle.from_json('{"type":"collection","entry":[{"resource":
{"resourceType":"Patient","id":"p-1"}}]}')
assert type(bundle.entry[0].resource) is Patient

# Typed usage narrows statically
bundle: Bundle[Patient] =
Bundle[Patient].model_validate(response.json())

Tests                    

- Added test_bundle.py — runtime tests for Bundle[T] type narrowing
- Added test_bundle_from_json in test_sdk.py — verifies bare
Bundle.from_json polymorphic dispatch
- Fixed client.search to use model_validate (dict) instead of
from_json (string)
- Fixed mypy strict error in client.py with # type: 
ignore[valid-type] for runtime generic subscript

[ryukzak]

Nice job!