Add unit tests for the event deduplication guard skipping already-processed events by transaction hash and event index

[Chucks1093]

Summary

The event ingestion pipeline can receive the same event more than once if the Horizon cursor is reset or a worker restarts mid-batch. The deduplication guard should detect already-processed events by their (txHash, eventIndex) composite key and skip them without writing a duplicate row.

Scope

• Test an event with a new (txHash, eventIndex) pair is inserted successfully
• Test an event with an already-seen (txHash, eventIndex) is skipped and returns { skipped: true }
• Test two events sharing the same txHash but different eventIndex values are both inserted
• Test the deduplication check is performed before any other processing (no side effects on skip)
• Assert the total event count in the database is unchanged after processing a duplicate

Acceptance Criteria

• New event pair inserted successfully
• Duplicate pair skipped with { skipped: true }
• Same tx hash with different event index treated as two distinct events
• No side effects (balance updates, logs) triggered on a skipped event
• Database event count unchanged after duplicate is skipped

ETA: 12 hours

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Coordinate on Telegram

[dair7]

I'd like to work on this issue.

I have experience writing backend unit tests and validating event processing logic. I'll add focused tests covering the deduplication guard for (txHash, eventIndex), verify that duplicate events are skipped with { skipped: true }, ensure events with the same transaction hash but different event indexes are processed independently, confirm the deduplication check happens before any side effects, and validate that the event count remains unchanged after processing duplicates. I'll submit a well-tested PR following the project's code style and testing practices.