test: add comprehensive test suite for all dns_utils modules

[tboy1337]

Summary

• Add a complete pytest-based test suite covering all dns_utils modules and the client/server application code
• 15 test files, ~6 200 lines of tests across unit, integration, and property-based scenarios

Test files added

File	Module under test
ests/conftest.py	Shared fixtures (loggers, parsers, mock I/O, sockets)
ests/test_arq.py	dns_utils.ARQ — state machine, data/control plane, retransmits
ests/test_client.py	client.py — MasterDnsVPNClient lifecycle and helpers
ests/test_compression.py	dns_utils.compression — all codec paths
ests/test_config_loader.py	dns_utils.config_loader — TOML loading, path resolution
ests/test_dns_balancer.py	dns_utils.DNSBalancer — server selection, health tracking
ests/test_dns_enums.py	dns_utils.DNS_ENUMS — enum completeness and values
ests/test_dns_packet_parser.py	dns_utils.DnsPacketParser — encode/decode for all encryption modes
ests/test_init.py	dns_utils.init — public API exports
ests/test_packet_queue_mixin.py	dns_utils.PacketQueueMixin — queue operations
ests/test_ping_manager.py	dns_utils.PingManager — latency tracking
ests/test_prepend_reader.py	dns_utils.PrependReader — stream buffering
ests/test_server.py	server.py — MasterDnsVPNServer handlers
ests/test_utils.py	dns_utils.utils — async networking helpers

Test approach

• pytest-asyncio with asyncio_mode = auto for all async tests
• hypothesis property-based tests for packet encoding/decoding and data-transform paths
• 30 s per-test timeout via pytest-timeout
• pytest-mock / unittest.mock for isolating I/O, sockets, and subprocesses

Notes

This branch also carries the source and config changes from the companion PRs (#1 chore/modernize-type-hints, #2 fix/arq-io-loop, #3 chore/dev-tooling-and-ci) in its diff since all work originated from a single branch. The test files themselves are the intended scope of this PR. Reviewing after the companion PRs are merged will reduce diff noise.