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fix(kafka): wait for topic metadata before subscribing

The E2E 'emit event' scenario still failed sporadically even with a
10s polling window: messages published to one specific topic were
never delivered while the other topics on the same consumer worked
fine.

The cause is that CreateTopics may return before the broker's
metadata cache knows the new topics' partition leaders (topic
creation propagates asynchronously, especially in ZooKeeper mode).
If the consumer joins at that moment, it gets no partition
assignment for those topics and never re-checks, silently
blackholing them.

Now the transporter polls the topic metadata after creation and only
starts the consumer once every topic reports elected partition
leaders (up to a 10s timeout).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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