Resolver tests perform iterative DNS resolution from root nameservers,
which can hang indefinitely. This broke make check on main.
Changes:
- Add -short and -timeout 30s flags to go test in make check
- Skip real-DNS integration tests when -short is set (via testContext helper)
- Context-canceled tests still run (no network needed)
- Also add -timeout 30s to make test target
Run integration tests explicitly with: go test -v -race ./internal/resolver/...
