feat(batch): add Kafka/MSK batch processing support

[shaked-lokits]

Issue number: closes #5362

Summary

Add support for processing Kafka and MSK events with partial batch failure reporting, leveraging the new ReportBatchItemFailures functionality that AWS recently enabled for Kafka event sources.

Changes

• Add EventType.Kafka to batch processor for unified MSK/self-managed Kafka support
• Implement _collect_kafka_failures() for composite identifiers (topic-partition + offset)
• Add _get_records_from_event() helper to flatten Kafka's nested record structure
• Add KafkaItemIdentifier TypedDict for type safety
• Add 14 functional tests covering sync/async processing, partial failures, and edge cases
• Update batch processing documentation with Kafka section and SAM template
• Update Kafka utility documentation with cross-reference to batch processing

User experience

Before: Users had to manually implement partial batch failure handling for Kafka/MSK events, including constructing the correct response format with composite identifiers.

After: Users can use the familiar batch processing API:

from aws_lambda_powertools.utilities.batch import BatchProcessor, EventType, process_partial_response

processor = BatchProcessor(event_type=EventType.Kafka)

def record_handler(record):
    # Process record
    return record.value

def lambda_handler(event, context):
    return process_partial_response(event=event, record_handler=record_handler, processor=processor, context=context)

---

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[boring-cyborg]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.73%. Comparing base (2c21763) to head (2ee2c90).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7941   +/-   ##
========================================
  Coverage    96.72%   96.73%           
========================================
  Files          278      278           
  Lines        13630    13646   +16     
  Branches      1084     1086    +2     
========================================
+ Hits         13184    13200   +16     
  Misses         327      327           
  Partials       119      119           

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[sonarqubecloud]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud