Debug and harden a driver-selection component

## Debugging Exercise: “Pick Dasher” You are given a small codebase for selecting a dasher (driver) using a heap/priority-queue–like structure. The current implementation is failing tests / behaving incorrectly. While debugging, you notice issues such as: - A required **constructor** was not implemented/initialized properly. - A method similar to `adjustKey()` has an **off-by-one** bug: it uses something like `size - 1` when `size` is already the correct last index (or vice versa). ### Tasks 1. Identify and fix the bug(s) so the selection logic works correctly. 2. Write your own **test cases** that reproduce the failure and verify the fix. ### Follow-up (production readiness) If this were production code, what changes would you make beyond the minimal bug fix? Consider: - input validation - error handling - logging/metrics - performance - maintainability (API design, naming, invariants, documentation) - test strategy (unit/integration/property-based) - rollout safety (feature flags, canary)