FEAT: add unit tests for gaslessUtxos collection

[GabrielTozatti]

Description

Implements unit tests for the UTXO pool collection covering all operations and concurrency scenarios.

Summary

• Added GaslessTestEnvironment setup utility with mongodb-memory-server for in-memory MongoDB
• Added unit tests for findAvailable() — 2 scenarios
• Added unit tests for reserve() — 5 scenarios including race conditions
• Added unit tests for release() — 3 scenarios
• Added unit tests for markSpent() — 3 scenarios
• Added unit tests for getStats() — 2 scenarios
• Added unit tests for releaseExpired() — 4 scenarios
• Added mongodb-memory-server as dev dependency for in-memory MongoDB in tests
• Added tsconfig-paths as dev dependency for path resolution in tests
• 26 tests passing

Checklist

• I reviewed my PR code before submitting
• I ensured that the implementation is working correctly and did not impact other parts of the app
• I mentioned the PR link in the task

[guimroque]

Code Review

Summary

This PR adds comprehensive unit tests for the gaslessUtxos collection, covering all CRUD operations and concurrency scenarios. The implementation uses mongodb-memory-server for isolated testing and includes proper race condition testing for the reservation system.

Strengths

• Excellent race condition testing in gaslessUtxos.test.ts:89-107 - properly validates atomic operations
• Clean test environment setup with proper lifecycle management in gaslessTestEnvironment.ts:10-25
• Comprehensive edge case coverage including null returns and expired reservation handling
• Good use of afterEach hooks for test isolation and cleanup

Issues

• 0 critical, 1 important, 1 suggestion