feat: Add configurable maxBatchSize to consumer.run()

[PabloReszczynski]

Allow users to configure the maximum batch size when using eachBatch handler instead of the hardcoded value of 32.

Changes:

• Add maxBatchSize parameter to ConsumerRunConfig type
• Update run() method to accept and validate maxBatchSize
• Default to 32 to maintain backwards compatibility
• Validate that maxBatchSize is a positive integer
• Update #runInternal to use configurable value
• Add JSDoc documentation for the new parameter
• Add comprehensive test coverage for the feature

The maxBatchSize parameter controls how many messages are included in a single batch when using the eachBatch handler. Users can now tune this based on their message size and processing requirements.

Tests verify default behavior, custom values, validation, large batch sizes, and that the parameter only affects eachBatch (not eachMessage).

Checklist

• Contains customer facing changes? Including API/behavior changes
• Did you add sufficient unit test and/or integration test coverage for this PR?
  • If not, please explain why it is not required

References

Addresses #286

Test & Review

Unit tested

Open questions / Follow-ups

This is my first PR on this repo, so any advice is welcome.

Review stakeholders

Probably @milindl since git blame points to him on the TODO.

[Copilot]

Pull Request Overview

Adds a configurable maxBatchSize option to consumer.run(), allowing users of eachBatch to control maximum batch size instead of the previously hardcoded 32.

• Introduces maxBatchSize validation and defaulting logic (intended default 32)
• Propagates maxBatchSize into #runInternal and updates tests to cover default, custom, large values, validation errors, and isolation from eachMessage
• Adds new tests validating behavior and error scenarios

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 4 comments.

File	Description
lib/kafkajs/_consumer.js	Adds JSDoc param, validation, and replaces hardcoded batch size with configurable value
test/promisified/consumer/consumeMessages.spec.js	Adds test cases for default/custom/large maxBatchSize, validation failures, and ensuring eachMessage unaffected

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