test: add regression test for indexer cursor advancing past failed ev…

[extolkom]

closes #846

Added regression test cursor_does_not_advance_past_failed_event_in_mixed_batch to backend/tests/soroban-event-worker.test.ts.

• Purpose: Pins correct behavior for issue [Testing] Indexer per-event failure path (batch continues, cursor advances past failed event) has no test #846 where a per-event failure in an indexer batch allows the cursor to advance past the failed event.
• Verification: Confirmed test fails against current main (AssertionError: expected 'cursor-event-3' not to be 'cursor-event-3').
• Git Branch: Committed and pushed to test/846-indexer-per-event-failure-cursor.