[Testing] Indexer per-event failure path (batch continues, cursor advances past failed event) has no test

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

tests/soroban-event-worker.test.ts covers idempotency/dedup and happy-path replays only. The failure branch in fetchAndProcessEvents - where one event throws but the batch continues and persists a later cursor - is the exact path behind the data-loss bug and is completely uncovered.

Acceptance criteria

• Add a test with a mocked getEvents batch where the first event's handler rejects and a later one resolves
• Assert the persisted IndexerState.lastCursor is NOT advanced beyond the failed event
• Assert the successful events are still written exactly once

Files to touch

• backend/tests/soroban-event-worker.test.ts
• backend/src/workers/soroban-event-worker.ts

Out of scope

• Real Soroban RPC integration