Chore: Add cross-platform integration test

[backkem]

Summary

Re-implement the CI integration test in a cross-platform way. The previous bash-based integration test
(integration-test.sh) was removed because:

1. mDNS discovery doesn't work reliably in GitHub Actions runners
2. Bash scripts don't seamlessly work cross platform (e.g. on Windows hosts)

Requirements

• Cross-platform: Should work identically on Linux, macOS, and Windows
• CI-compatible: Must work in GitHub Actions runners
• Reliable mDNS: Use Docker networking to ensure mDNS discovery works consistently

Proposed Approach

1. Rewrite test runner in Python for cross-platform compatibility
2. Use Docker Compose with a compose file for consistent local/CI behavior
3. Consider docker-in-docker setup for Actions runners

Test Scenario

Start app-receiver and app-sender with:

• Custom mDNS port to avoid conflicts with system mDNS
• Unique receiver name per test run
• PSK-based authentication
• Verify sender discovers receiver via mDNS and connects successfully

References

• PR Expand CI workflows #6 comment: Expand CI workflows #6 (comment)