test: implement indexer_runner transaction isolation validation (#185)

[Davoski1]

Description

Closes #185

This PR addresses the requirement to ensure that indexer_runner operations run inside SQLite transaction blocks to protect data consistency under load.

During investigation, we confirmed that the primary write path for the main indexer event poller execution loop (insertEventBatch called from pollEvents) is already safely wrapped in a strict synchronous db.transaction() block provided by better-sqlite3.

To fulfill the issue's validation and testing requirements, this PR focuses on formally proving this transactional safety by introducing explicit test coverage that asserts the rollback behavior.

Changes Included

• Added: __tests__/indexer-runner-transaction.test.ts to validate transaction isolation during polling.
• Validation: Added a specific test case that simulates a constraint/execution failure mid-transaction and asserts that the last_ledger_sequence pointer does not advance (i.e., verifying that partial database updates rollback fully on execution failures).

Testing & Verification

• Confirmed database updates rollback fully on execution failures via the new test suite.
• Verified that all existing project tests continue to compile and pass successfully.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes