[PR] [fix][client] Send the pending batch before a non-batched message so deduplication does not drop it [pulsar-client-cpp]

["gusteycamargo (via GitHub)" <gi...@apache.org>]

gusteycamargo opened a new pull request, #616:
URL: https://github.com/apache/pulsar-client-cpp/pull/616

   Fixes #615
   
   ### Motivation
   
   When batching is enabled, a message that cannot be batched (e.g. a delayed message) is sent immediately, while the messages already in the batch container still wait for the batch to be flushed. The non-batched message took the next sequence id, so it reaches the broker before messages with lower sequence ids. With deduplication enabled, the broker then drops the whole batch as a duplicate, while the producer receives `ResultOk` for every message.
   
   The Java client avoids it by flushing the batch container before sending any non-batched message (`ProducerImpl#processOpSendMsg`).
   
   ### Modifications
   
   - In `ProducerImpl::sendAsync`, before sending a message that cannot be added to the batch, send the pending batch (`batchMessageAndSend().complete()`), the same way the producer already does when the batch container has no room for the next message.
   
   ### Verifying this change
   
   - [ ] Make sure that the change passes the CI checks.
   
   This change added tests and can be verified as follows:
   
   - Added `ClientDeduplicationBatchingTest.testBatchedMessagesBeforeDelayedMessage`, parameterized for `DefaultBatching` and `KeyBasedBatching`: with deduplication enabled on the namespace, it sends 10 batched messages asynchronously and then a delayed one, and reads the topic from the earliest position. Without the fix only 1 of the 11 messages is stored, with it all 11 are.
   
   ### Documentation
   
   - [ ] `doc-required`
   (Your PR needs to update docs and you will update later)
   
   - [x] `doc-not-needed`
   (Bug fix in the producer, no API change)
   
   🤖 Generated with [Claude Code](https://claude.com/claude-code)
   


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