Support args of custom types in dspy mcp tool

[chenmoneygithub]

Currently if an MCP tool's arg is of custom type, then the tool.args doesn't reflect the json schema correctly. For example:

from mcp.server.fastmcp import FastMCP
from pydantic import BaseModel

mcp = FastMCP("test")


class Profile(BaseModel):
    name: str
    age: int


class Account(BaseModel):
    profile: Profile
    account_id: str

@mcp.tool()
def get_account_name(account: Account):
    """This extracts the name from account"""
    return account.profile.name


if __name__ == "__main__":
    mcp.run()

will have the args be like {'account': {'$ref': '#/$defs/Account'}}, which doesn't have sufficient information for the LM to generate right args. This PR fixes it by explicitly resolving the refs.