Fix OpenAPI default, const, and $ref parameter support

[chassing]

Summary

• default support: generate_class_properties now reads "default" from OpenAPI property definitions and renders the value as a Python literal via repr(). Fields with both a default and an alias use pydantic.Field(default=..., alias="..."). Uses "default" in arg_details (not truthiness) so falsy defaults like [], 0, False, "", and null are handled correctly.
• const support: get_type now checks for "const" and generates typing.Literal[<value>]. This is required for Pydantic discriminated unions to work — without Literal, Pydantic raises PydanticUserError: Model needs field to be of type Literal.
• $ref in parameters: Parameters referencing schemas via $ref (e.g. enums like ActionStatus) now resolve to schemas.X in client.py instead of bare forward references with no import.
• Double-nullable fix: anyOf: [$ref, {type: null}] parameters no longer produce X | None | None — existing nullability is detected and not duplicated.
• schema_to_dict compat layer: Extracts both "default" and "const" from cicerone's __pydantic_extra__, which were previously silently dropped during spec parsing.

Before:

# schemas.py — defaults ignored, const ignored
class Config(pydantic.BaseModel):
    tags: list[str]       # spec says default: []
    count: int            # spec says default: 0

class NotificationAddUser(pydantic.BaseModel):
    action: str           # spec says const: "add-user" — breaks discriminated unions
    message: str

# client.py — broken import, double None
def action_list(
    status: ActionStatus | None | None = None,  # NameError: ActionStatus not imported
) -> schemas.ResponseActionList: ...

After:

# schemas.py
class Config(pydantic.BaseModel):
    tags: list[str] = []
    count: int = 0

class NotificationAddUser(pydantic.BaseModel):
    action: typing.Literal["add-user"] = "add-user"  # discriminated unions work
    message: str

# client.py
def action_list(
    status: typing.Optional[schemas.ActionStatus] = None,  # correct import, single None
) -> schemas.ResponseActionList: ...

Test plan

• test_generate_class_properties_with_defaults — list [], int 0, bool False, string "", null defaults
• test_generate_class_properties_default_with_alias — field with alias + default uses pydantic.Field
• test_generate_class_properties_no_required_array_with_defaults — schema with no required array, all fields have explicit defaults
• test_generate_class_properties_with_const — const generates typing.Literal
• test_generate_class_properties_const_with_default — const + default generates Literal type with default value
• test_generate_class_properties_const_with_alias — const + alias uses pydantic.Field
• test_resolve_forward_refs_for_client — forward refs replaced with schemas.X
• test_strip_none_from_type — handles Optional, Union with None, pipe None, plain types
• test_clients_generator_resolves_schema_refs_in_parameters — anyOf: [$ref, null] produces typing.Optional[schemas.X] without double None
• End-to-end: specs with discriminator + enum ref parameters generate correct, loadable code
• Full test suite passes (559/559, excluding pre-existing aiohttp failures)

[phalt]

@chassing thanks for this, I am going to try and look into it tomorrow (Friday) and get this merged and shipped. Please could you update the CHANGELOG.md files with one-sentence notes of the fixes? Feel free to make a new version 2.2.0

[phalt]

Rebasing so CI can run.

[phalt]

This is great. I am working to fix a few more bugs on another issue - I think you actually fix one of mine here! So I'll merge your stuff and then continue with my work before I do the final 2.2.0 release :)