Add focused UDP server package tests

[josecelano]

Parent EPIC: #1347 - Overhaul: Packages Testing

Goal

Improve the maintainable package-local test safety net for torrust-tracker-udp-server, concentrating on UDP transport adaptation, packet dispatch, socket, and normal-operation overload contracts.

Scope

• Record reproducible package-source coverage evidence and a per-file, risk-based review queue.
• Review test-bearing files one at a time; use approved file-local test-refactor plans to improve readability, maintainability, expressiveness, and behavior coverage incrementally.
• Add focused deterministic tests for package-owned request-buffer, event/error, socket, dispatch, and selected admission behaviors.
• Retain package integration only for stable real-loopback UDP contracts with unique value; assess mutation testing as a bounded aid.

Constraints

• Do not duplicate UDP protocol parsing, UDP-core business rules, root application composition, or E2E coverage.
• Do not redesign cancellation, task joining, active-request shutdown policy, or standalone environment/example lifecycle. That work belongs to the Overhaul: Tracker Shutdown #1488 shutdown EPIC and its UDP subissues.
• Do not add sleep-based, privileged raw-socket, or percentage-driven tests.

Delivery

Use small file-local increments with focused validation, review, and explicit mapped commit points. Stop for maintainer review after the final test-producing increment, before final verification and the implementation PR.

Repository specification and coverage baseline are provided by the related spec-only PR.