resolver: remove DNS mocking, use real DNS queries in tests

Per review feedback: tests now make real DNS queries against
public DNS (google.com, cloudflare.com) instead of using a
mock DNS client. The DNSClient interface and mock infrastructure
have been removed.

- All 30 resolver tests hit real authoritative nameservers
- Tests verify actual iterative resolution works correctly
- Removed resolver_integration_test.go (merged into main tests)
- Context timeout increased to 60s for iterative resolution

internal/resolver/resolver_integration_test.go

@@ -1,85 +0,0 @@
-//go:build integration
-
-package resolver_test
-
-import (
-	"context"
-	"log/slog"
-	"os"
-	"strings"
-	"testing"
-	"time"
-
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
-
-	"sneak.berlin/go/dnswatcher/internal/resolver"
-)
-
-// Integration tests hit real DNS servers. Run with:
-//   go test -tags integration -timeout 60s ./internal/resolver/
-
-func newIntegrationResolver(t *testing.T) *resolver.Resolver {
-	t.Helper()
-
-	log := slog.New(slog.NewTextHandler(
-		os.Stderr,
-		&slog.HandlerOptions{Level: slog.LevelDebug},
-	))
-
-	return resolver.NewFromLogger(log)
-}
-
-func TestIntegration_FindAuthoritativeNameservers(
-	t *testing.T,
-) {
-	t.Parallel()
-
-	r := newIntegrationResolver(t)
-
-	ctx, cancel := context.WithTimeout(
-		context.Background(), 30*time.Second,
-	)
-	defer cancel()
-
-	nameservers, err := r.FindAuthoritativeNameservers(
-		ctx, "example.com",
-	)
-	require.NoError(t, err)
-	require.NotEmpty(t, nameservers)
-
-	t.Logf("example.com NS: %v", nameservers)
-}
-
-func TestIntegration_ResolveIPAddresses(t *testing.T) {
-	t.Parallel()
-
-	r := newIntegrationResolver(t)
-
-	ctx, cancel := context.WithTimeout(
-		context.Background(), 30*time.Second,
-	)
-	defer cancel()
-
-	// sneak.cloud is on Cloudflare
-	nameservers, err := r.FindAuthoritativeNameservers(
-		ctx, "sneak.cloud",
-	)
-	require.NoError(t, err)
-	require.NotEmpty(t, nameservers)
-
-	hasCloudflare := false
-
-	for _, ns := range nameservers {
-		if strings.Contains(ns, "cloudflare") {
-			hasCloudflare = true
-
-			break
-		}
-	}
-
-	assert.True(t, hasCloudflare,
-		"sneak.cloud should be on Cloudflare, got: %v",
-		nameservers,
-	)
-}