fix: remove dead Pydantic Field and model_config from RouterConfig plain class

[totto]

What

RouterConfig is a plain Python class with a custom __init__, but it declared two Pydantic constructs that have no effect:

class RouterConfig:  # not BaseModel
    routes: List[Route] = Field(default_factory=list)  # ← FieldInfo, never processed
    model_config: ClassVar[ConfigDict] = ConfigDict(...)  # ← never applied

Without BaseModel inheritance, Pydantic never processes Field() — the class attribute is set to a raw FieldInfo object (which __init__ immediately overwrites with the actual list). ConfigDict is similarly inert.

Why it matters

• Field(default_factory=list) sets the class attribute to a FieldInfo object, not a list. If anything ever reads RouterConfig.routes before __init__ runs (e.g. introspection, subclass), it gets a FieldInfo instead of a list.
• The dead declarations mislead contributors into thinking RouterConfig is a Pydantic model.

Fix

Keep the type annotation routes: List[Route] for documentation value. Remove Field() and model_config entirely.

class RouterConfig:
    routes: List[Route]

    def __init__(self, routes: List[Route] = [], ...):