fix(producer): prevent send requests with 0 as num_messages

[BewareMyPower]

fixes #376

[Copilot]

Pull request overview

This PR fixes issue #376 by preventing the producer from sending messages with num_messages_in_batch = 0 to the Pulsar broker. When send_batch() is called on a partitioned producer with no pending messages in some partitions, the fix ensures those partitions signal the flush completion without attempting to send an empty batch.

Key changes:

• Added early return in message_send_loop when counter is 0 after extracting flush item
• Created a new test_utils module to consolidate common test helper functions
• Added test coverage for flushing on partitioned topics where some partitions have no messages

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/producer.rs	Adds the core fix to prevent sending messages with 0 count; refactors test helper usage to use new test_utils module
src/test_utils.rs	New module containing new_pulsar() and create_partitioned_topic() test helper functions extracted from producer tests
src/lib.rs	Adds test_utils module; refactors existing tests to use test_utils helpers; adds new test for flush on partitioned topics
src/reader.rs	Reorders imports to follow consistent style (std, external crates, then internal modules)

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