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Description

Documents the Kafka transactions support added to the Kafka pubsub component in dapr/components-contrib#4508 (targeted at Dapr 1.19):

• setup-apache-kafka.md: five new spec metadata fields (producerTransactionsEnabled, consumerTransactionsEnabled, transactionalIdPrefix, consumerIsolationLevel, transactionTimeout) and a new "Kafka transactions and exactly-once processing" section covering transactional publishing, committed-only consumption, and the exactly-once consume-transform-produce pattern — the __txnToken echo contract (HTTP/gRPC/bulk), failure and redelivery semantics, scope of the guarantee, transaction timeout guidance, and scaling/zombie-fencing notes. The __txnToken entry is also added to the per-call metadata fields and consumer-side special headers lists.
• kafka.md (bindings): the producer-side transaction fields and consumerIsolationLevel, plus a note that consumerTransactionsEnabled is rejected at init for bindings and why (pubsub-only feature).

This is the documentation PR required by dapr/components-contrib#4508; both should ship in the same release. Unchecked checklist items (tabpane, images) are not applicable: the changes are reference tables, YAML component snippets, and one single-command HTTP example on pages that don't use tabpane for these.

Issue reference

dapr/components-contrib#4301