Use isinstance() instead of type() == for exception type checking

Replace `type(e) == litellm.exceptions.RateLimitError` with
`isinstance(e, litellm.exceptions.RateLimitError)` for proper exception
type checking.

While technically different behavior (isinstance() supports
inheritance while type() == requires exact match), isinstance() is the
correct and more robust approach for exception handling. If litellm
introduces subclasses of RateLimitError in the future, isinstance() will
correctly catch them.