Add LLM-assisted retries

[jackmpcollins]

Add max_retries param to prompt and chatprompt decorators for retries using the LLM to fix its own errors.

Example of using Pydantic validator to reject an LLM output and have it retried.

from typing import Annotated

from magentic import OpenaiChatModel, UserMessage, prompt
from magentic.chat_model.message import AssistantMessage, Message
from pydantic import AfterValidator, BaseModel


def assert_is_ireland(v: str) -> str:
    assert v == "Ireland", "Country must be Ireland"
    return v


class Country(BaseModel):
    name: Annotated[str, AfterValidator(assert_is_ireland)]
    capital: str


@prompt("Return a country", max_retries=3)
def get_country() -> Country: ...


get_country()

07:13:45.872 Calling prompt-function get_country
07:13:45.888   LLM-assisted retries enabled. Max 3
07:13:45.935     Chat Completion with 'gpt-4o' [LLM]
07:13:46.397     streaming response from 'gpt-4o' took 0.14s [LLM]
07:13:46.399     Retrying Chat Completion. Attempt 1
07:13:46.411     Chat Completion with 'gpt-4o' [LLM]
07:13:46.938     streaming response from 'gpt-4o' took 0.16s [LLM]

Country(name='Ireland', capital='Dublin')

Closes #166
Related to #210