feat: support structured output retry middleware

[sydney-runkle]

• attach the latest AIMessage to all StructuredOutputErrors so that relevant middleware can use as desired
• raise StructuredOutputError from ProviderStrategy logic in case of failed parsing (so that we can retry from middleware)
• added a test suite w/ example custom middleware that retries for tool + provider strategy

Long term, we could add our own opinionated structured output retry middleware, but this at least unblocks folks who want to use custom retry logic in the short term :)

class StructuredOutputRetryMiddleware(AgentMiddleware):
    """Retries model calls when structured output parsing fails."""

    def __init__(self, max_retries: int) -> None:
        self.max_retries = max_retries

    def wrap_model_call(
        self, request: ModelRequest, handler: Callable[[ModelRequest], ModelResponse]
    ) -> ModelResponse:
        for attempt in range(self.max_retries + 1):
            try:
                return handler(request)
            except StructuredOutputError as exc:
                if attempt == self.max_retries:
                    raise

                ai_content = exc.ai_message.content
                error_message = (
                    f"Your previous response was:\n{ai_content}\n\n"
                    f"Error: {exc}. Please try again with a valid response."
                )
                request.messages.append(HumanMessage(content=error_message))