fix: implement SQS partial batch failure handling

[ngoyal16]

Fixes #3

Summary of Changes

• Partial Batch Failure Reporting: Modified the Lambda handler signature to return events.SQSEventResponse containing BatchItemFailures.
• Per-Record Processing: Extracted message processing logic into processMessage and processSQSEvent. If an individual record fails, its MessageId is appended to BatchItemFailures while allowing other records in the SQS batch to proceed without reprocessing previously written records.
• CloudFormation Configuration: Updated the EventSourceMapping in deploy/cloudformation/lambda.yaml with FunctionResponseTypes: ['ReportBatchItemFailures'] so AWS Lambda properly respects partial failure responses.
• Mockable Client Interfaces: Defined S3Client and KMSClient interfaces to decouple AWS SDK calls and enable clean unit test mocking.
• Parquet Tag Fix: Fixed conflicting dict,plain encoding tags on DatabaseActivityEvent fields which were causing a runtime panic in parquet-go.
• Unit Test Coverage: Added comprehensive unit tests in main_test.go covering:
  • 2 records where first succeeds and second fails -> only second message in BatchItemFailures
  • All records succeed -> empty BatchItemFailures
  • All records fail -> all message IDs in BatchItemFailures
  • Empty batch handling
  • S3 and KMS error paths
  • End-to-end payload processing with encryption and decompression