[lankeethebeast]

Closes #226

Summary

assignmentService had no tests. This adds 22 unit tests in tests/assignmentService.test.ts, which follows the repo's flat tests/ layout. The issue names backend/tests/unit/services/, but that directory doesn't exist.

Deliveries, escrows and driver positions are stored in MongoMemoryServer. That means proximity search runs the real $geoNear pipeline against the real 2dsphere indexes, and assignment goes through the real deliveryService.assignDriver escrow guard. Only withLock (Redlock/Redis) is replaced, by an in-process lock fake that records acquire/release and can simulate contention.

What is covered

• Nearest driver: closest available driver wins, and unavailable or offline drivers nearer to the pickup are ignored. Only the winner is marked on_delivery.
• Radius expansion: base → ×2 → ×4 until a driver is in range. The search stops after ASSIGNMENT_RADIUS_EXPANSION_STEPS, never goes past DRIVER_PROXIMITY_MAX_RADIUS_M, and searches the cap only once.
• Contention: a driver claimed elsewhere between search and claim is skipped and the runner-up is assigned. Two deliveries racing for one driver produce exactly one assignment. A held delivery lock rejects without touching any driver.
• releaseClaim: when the escrow guard rejects (PENDING, DISPUTED, or no escrow), the driver goes back to isAvailable: true / online with no currentDeliveryId, and the lock is released. If the rollback write itself fails, the original error still surfaces.
• autoAssignPendingDeliveries: only FUNDED, driverless deliveries are picked, oldest first, within the batch limit. Locked and failing deliveries are skipped without stopping the sweep, and the { attempted, assigned } counts are checked.
• Validation: malformed id (rejected before the lock is taken), 404, a delivery that already has a driver, missing pickup coordinates.

Verification

npx jest tests/assignmentService.test.ts --coverage --collectCoverageFrom=src/services/assignmentService.ts
Tests:       22 passed, 22 total
assignmentService.ts | 100 % Stmts | 91.66 % Branch | 100 % Funcs | 100 % Lines

eslint and prettier are clean on the new file.

Notes

• When every expansion step runs out before the cap, the "not assigned" result reports radiusMeters as the next, unsearched radius. For example, with 3 steps it searches 5/10/20/40 km but reports 50 km. The tests don't lock that value in. Happy to fix it in a follow-up if you'd like.
• releaseClaim resets availability only and never sets currentDeliveryId. The tests assert that it stays null.

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