test(request): improve coverage across package (#7307)

Add tests for previously untested functions:
- edns0.go: test supportedOptions function
- request.go: test address methods, protocol handling, and EDNS0
  options
- writer.go: test ScrubWriter implementation

Improves overall package test coverage from 39.5% to 77.8%.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

request/edns0_test.go

@@ -0,0 +1,50 @@
+package request
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestSupportedOptions(t *testing.T) {
+	tests := []struct {
+		name     string
+		options  []dns.EDNS0
+		expected int
+	}{
+		{
+			name:     "empty options",
+			options:  []dns.EDNS0{},
+			expected: 0,
+		},
+		{
+			name: "all supported options",
+			options: []dns.EDNS0{
+				&dns.EDNS0_NSID{},
+				&dns.EDNS0_EXPIRE{},
+				&dns.EDNS0_COOKIE{},
+				&dns.EDNS0_TCP_KEEPALIVE{},
+				&dns.EDNS0_PADDING{},
+			},
+			expected: 5,
+		},
+		{
+			name: "mixed supported and unsupported options",
+			options: []dns.EDNS0{
+				&dns.EDNS0_NSID{},
+				&dns.EDNS0_LOCAL{Code: 65001}, // unsupported code
+				&dns.EDNS0_PADDING{},
+			},
+			expected: 2,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			result := supportedOptions(tc.options)
+			if len(result) != tc.expected {
+				t.Errorf("Expected %d supported options, got %d", tc.expected, len(result))
+			}
+		})
+	}
+}