Send writers messages as one batch

[vgvoleg]

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
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• Documentation content changes
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What is the current behavior?

Issue Number: N/A

What is the new behavior?

Other information

[Copilot]

Pull request overview

This PR updates the topic writer’s asyncio reconnector to send queued messages in larger batches (instead of one gRPC write per message), and extends test coverage to validate batching and request-splitting behavior.

Changes:

• Batch sends in WriterAsyncIOReconnector._send_loop by draining _new_messages and calling writer.write() once per batch.
• Update asyncio reconnector tests to expect batched sends and batched re-sends after reconnect.
• Add unit tests for _split_messages_for_send() and messages_to_proto_requests() (codec-run grouping + max gRPC size splitting).

Reviewed changes

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

File	Description
ydb/_topic_writer/topic_writer_asyncio.py	Batch messages in the reconnector send loop to reduce per-message write calls.
ydb/_topic_writer/topic_writer_asyncio_test.py	Adjust expectations for batched sends; add batching-focused tests.
ydb/_topic_writer/topic_writer_test.py	Add unit tests for message grouping/splitting into proto write requests.

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