[REQ] [python-fastapi] Support optional authentication

Is your feature request related to a problem? Please describe.

OAS lets you declare a service with optional authentication, for example: https://gist.github.com/micolous/2593789b5b797015dc32ddf049a028a1#file-optional_auth_openapi-yaml-L69-L72

With the Python-FastAPI generator (v7.9.0) this results in:

# security_api.py
oauth2_code = OAuth2AuthorizationCodeBearer(
    authorizationUrl="https://auth.example.com/authorize",
    tokenUrl="https://auth.example.com/token",
    refreshUrl="",
    scopes={
        "read:api": "API read access",
        "special:api": "API special access",
        "write:api": "API write access",
    }
)

def get_token_oauth(
    security_scopes: SecurityScopes, token: str = Depends(oauth2_code)
) -> TokenModel:
    ...

# apis/default_api.py
@router.get(
    "/default_optional_auth",
    responses={
        200: {"model": str, "description": "Information about the currently logged in user."},
    },
    tags=["default"],
    summary="Method with optional authentication, from a default policy.",
    response_model_by_alias=True,
)
async def default_optional_auth(
    token_oauth: TokenModel = Security(
        get_token_oauth, scopes=["read:api"]
    ),
) -> str:
    if not BaseDefaultApi.subclasses:
        raise HTTPException(status_code=500, detail="Not implemented")
    return await BaseDefaultApi.subclasses[0]().default_optional_auth()

FastAPI's OAuth2 flows (like OAuth2AuthorizationCodeBearer) default to auto_error=True, which causes them to reject any requests which do not include authentication.

Describe the solution you'd like

With the service definition provided, there should be multiple OAuth2AuthorizationCodeBearer instances:

• default_optional_auth and path_level_optional_auth should have an instance with auto_error=False
• required_auth should have an instance with auto_error=True

The API implementations (in apis/default_api.py should declare token_oauth: Optional[TokenModel] = Security(... for default_optional_auth and path_level_optional_auth.

This will also need to end up in the base APIs, once #20048 is fixed.

Describe alternatives you've considered

Additional context

Start with the Python-FastAPI generator paths that produce security_api.py and apis/default_api.py, using the optional_auth_openapi.yaml service definition from the issue as the reproduction input. Compare generated handling for default_optional_auth, path_level_optional_auth, and required_auth, including the referenced base APIs and #20048 dependency. Done means optional operations allow unauthenticated requests while required_auth still enforces authentication, with matching Optional[TokenModel] declarations.