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/writer_test.go

@@ -0,0 +1,51 @@
+package request
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+// mockResponseWriter implements dns.ResponseWriter interface for testing
+type mockResponseWriter struct {
+	test.ResponseWriter
+	lastMsg *dns.Msg
+}
+
+func (m *mockResponseWriter) WriteMsg(msg *dns.Msg) error {
+	m.lastMsg = msg
+	return nil
+}
+
+func TestScrubWriter(t *testing.T) {
+	req := new(dns.Msg)
+	req.SetQuestion("example.com.", dns.TypeA)
+	req.SetEdns0(4096, true)
+
+	mock := &mockResponseWriter{}
+	sw := NewScrubWriter(req, mock)
+
+	// Create a large response message
+	resp := new(dns.Msg)
+	resp.SetReply(req)
+
+	// Add a lot of records to make it large
+	for i := 1; i < 100; i++ {
+		resp.Answer = append(resp.Answer, test.A(
+			fmt.Sprintf("example.com. 10 IN A 10.0.0.%d", i)))
+	}
+
+	// Write the message through ScrubWriter
+	err := sw.WriteMsg(resp)
+	if err != nil {
+		t.Errorf("Expected no error, got: %v", err)
+	}
+
+	// Verify that ScrubWriter called methods properly
+	if mock.lastMsg == nil {
+		t.Fatalf("Expected WriteMsg to be called with a message")
+	}
+}