fix(kafka): fix use-after-free on task cancellation during kafka producer Send

[disaykin]

Fixes a Use-After-Free vulnerability in the asynchronous Kafka Producer API when a task is cancelled before librdkafka flushes its queue.

The kafka::Producer::Send API accepts parameters as views (string_view, zstring_view, HeaderViews). These views point to data owned by the caller. Inside Send, raw pointers from these views are passed down to librdkafka, which stores them in its internal asynchronous queues for background delivery.

If a client connection drops, a cascade cancellation of tasks occurs. This leads to a premature return from kafka::Producer::Send, causing the caller to destroy the original payload memory. However, librdkafka still retains these pointers and attempts to access them during subsequent network flushes to the broker, resulting in memory corruption.

To fix this, we ensure that Producer::Send is blocked from returning prematurely and cannot exit until the background SendImpl completes its interaction with librdkafka.

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[disaykin]

@apolukhin прошу ревью. проблему с установкой кафки в тестах я починил (там протух урл), но вот что делать с флапающим тестом на постгрю - не знаю:

[ RUN      ] PoolTests/PostgrePool.PoolServerUnavailable/Sync
userver/postgresql/src/storages/postgres/tests/pool_pgtest.cpp:313: Failure
Expected equality of these values:
  0
  stats.connection.active
    Which is: 1

[  FAILED  ] PoolTests/PostgrePool.PoolServerUnavailable/Sync, where GetParam() = 4-byte object <00-00 00-00> (2013 ms)

Но это точно не относится к моему фиксу.

Также проблема компиляции с rabbitmq на debian12 привнесена не мной/