KIP-1118 — Add Deadlock Protection on Producer Network Thread

Accepted Kafka 4.1 Producer

Adds deadlock detection to `KafkaProducer` so that calling `Producer#flush()` from within a `send()` callback throws `KafkaException` instead of blocking indefinitely. Invoking `flush()` from the I/O thread that executes send callbacks creates an irresolvable circular wait, causing application hangs that are difficult to diagnose.

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