fix: stop poison-pill batches from freezing indexer cursor

[ponmileleke54-dev]

Summary

Fixes the poison-pill batch regression where a single malformed event in a batch caused the indexer cursor to freeze and prevented later valid events in the same batch from being processed on the next poll.

Changes

• Update the Soroban event worker to advance the cursor based on the most recent successfully processed event instead of stopping at the first failed event in a batch.
• Add a regression test covering a batch with an early malformed event followed by valid later events.

Verification

RUN v3.2.7 /workspaces/flowfi/backend

✓ tests/soroban-event-worker.test.ts (13 tests) 130ms

Test Files 1 passed (1)
Tests 13 passed (13)
Start at 11:35:49
Duration 791ms (transform 215ms, setup 0ms, collect 449ms, tests 130ms, environment 0ms, prepare 67ms)

This keeps later valid events from being reprocessed forever while still preserving the no-retry behavior for the failed event itself.

closed #1292

[K1NGD4VID]

resolve conflict on this issue @ponmileleke54-dev

[K1NGD4VID]

resolve conflict on this issue @ponmileleke54-dev