test(kafka-python): cover producer span error recording in send() (#4935)

The behavioral fix originally proposed here landed on main via #4858,
which moved the wrapped send() call inside the producer span context.
Keep the regression tests so that errors raised by KafkaProducer.send()
are guaranteed to be recorded on the producer span.

Assisted-by: Claude Fable 5

Co-authored-by: Emídio <9735060+emdneto@users.noreply.github.com>