Add Python code generator using pydantic (python subcommand)

[Copilot]

Adds a python subcommand to dcrpcgen that generates a typed Python JSON-RPC client from an OpenRPC schema, using pydantic for type validation and deserialization.

Generated output (3 files)

types.py

• Object schemas → BaseModel with ConfigDict(populate_by_name=True) and explicit Field(alias="camelCase") per field (avoids pydantic's alias_generator which misroundtrips names like e2eeAvail)
• String enums → StrEnum with UPPER_CASE = "original_value"
• Discriminated unions → Annotated[Union[VariantA, VariantB], Field(discriminator="kind")]; each variant carries kind: Literal["value"]
• All field names in snake_case

class AccountConfigured(BaseModel):
    model_config = ConfigDict(populate_by_name=True)
    kind: Literal["Configured"]
    color: str
    id: int
    display_name: Optional[str] = Field(default=None, alias="displayName")

Account = Annotated[Union[AccountConfigured, AccountUnconfigured], Field(discriminator="kind")]

rpc.py

• Synchronous Rpc class; method params in snake_case
• Non-None returns use TypeAdapter(T).validate_python(result)
• Pydantic model params serialized via _wrap() → model.model_dump(by_alias=True)

def get_contact(self, account_id: int, contact_id: int) -> Contact:
    return TypeAdapter(Contact).validate_python(
        self.transport.call("get_contact", account_id, contact_id)
    )

transport.py

• IOTransport over a deltachat-rpc-server subprocess (adapted from draft PR add python generator #2); returns raw Python data for pydantic to process upstream
• Abstract RpcTransport base class for custom transports

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