Tool default values aren't detected when BaseModel is supplied

Here's a minimal example where the default for `b` isn't respected. ```python from chatlas import Tool from pydantic import BaseModel, Field class AddParams(BaseModel): """Add two numbers together.""" a: int = Field(description="The first number to add.") b: int = Field(description="The second number to add.") def add(a, b=1) -> int: return a + b tool = Tool.from_func(add, model=AddParams) func = tool.schema["function"] params = func.get("parameters", {}) props = params["properties"] assert props["b"]["default"] == 1 ``` Note that this isn't an issue if either: 1. The `default` is supplied to `Field` 2. The `AddParams` model isn't supplied.