fix: handle HTTP-date in Retry-After header during 429 retries

[Yugenjr]

fixes #386

This PR wraps the conversion to float in a try...except ValueError block. If parsing the header fails (due to it being a date string), the provider falls back safely to the pre-calculated exponential backoff delay instead of crashing.

Files Involved
hiring-agent/models.py
(inside OpenAICompatibleProvider.chat)

Changes Made :

        if response.status_code == 429 and attempt < MAX_RETRIES - 1:
            retry_after = response.headers.get("Retry-After")
            exp_delay = min(BASE_DELAY * (2 ** attempt), MAX_DELAY)
            try:
                delay = float(retry_after) if retry_after else exp_delay
            except ValueError:
                delay = exp_delay