API: add python client option (fhirpy|none), default fhirpy; deprecate fhirpyClient

[ryukzak]

Generator/library change extracted from the python-fhirpy-default work (per review): introduce a client option for the Python generator and make fhirpy the default, plus a type-checking fix for fhirpy + snake_case models.

.python({ client })

Replace the boolean fhirpyClient with:

client?: "fhirpy" | "none"   // default: "fhirpy"

• "fhirpy" — models extend FhirpyBaseModel (fhirpy AsyncFHIRClient compatible).
• "none" — plain Pydantic models, no client-specific code.
• fhirpyClient is kept and @deprecated — it still works (logs a one-line warning and is translated: true→"fhirpy", false→"none"). Explicit client wins.
• Default is now "fhirpy" — callers that want plain models opt in with client: "none".

.python({ fhirpyClient: true })   // deprecated
.python({ client: "fhirpy" })     // or just omit it — fhirpy is the default
.python({ client: "none" })       // plain models

fhirpy + snake_case type-checking fix

FhirpyBaseModel now declares a TYPE_CHECKING-only settable resourceType: str = "". fhirpy's TResource is bound to a ResourceProtocol with a settable resourceType; snake_case models expose the discriminator as resource_type (alias resourceType) and only set resourceType at runtime via __pydantic_init_subclass__, which mypy can't see — so fhirpy calls failed type-checking. The annotation is settable (satisfies the protocol bound + class access), defaulted (so the pydantic mypy plugin treats it as optional, not a required ctor arg), and runtime-invisible.

Tests

• Existing python writer tests made explicit (client: "none") to keep testing plain generation.
• New tests: default → fhirpy (FhirpyBaseModel + fhirpy_base_model.py present), "none" → plain (absent), deprecated fhirpyClient: true → fhirpy.
• tsc + biome clean; no snapshot changes.

Foundation for the example PRs (#185 / #186); merges independently to main.