test(dnsserver): improve core/dnsserver test coverage (#7317)

Add comprehensive tests for multiple components including server blocks
inspection, configuration handling, DoH/DoQ writers, and server startup
functions. Increases overall test coverage from 27% to 38.4% with
particular focus on register.go, https.go, quic.go, and config.go.

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

core/dnsserver/config_test.go

@@ -0,0 +1,67 @@
+package dnsserver
+
+import (
+	"testing"
+
+	"github.com/coredns/caddy"
+)
+
+func TestKeyForConfig(t *testing.T) {
+	tests := []struct {
+		name          string
+		blockIndex    int
+		blockKeyIndex int
+		expected      string
+	}{
+		{"zero_indices", 0, 0, "0:0"},
+		{"positive_indices", 1, 2, "1:2"},
+		{"larger_indices", 10, 5, "10:5"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			result := keyForConfig(tc.blockIndex, tc.blockKeyIndex)
+			if result != tc.expected {
+				t.Errorf("Expected %s, got %s for blockIndex %d and blockKeyIndex %d",
+					tc.expected, result, tc.blockIndex, tc.blockKeyIndex)
+			}
+		})
+	}
+}
+
+func TestGetConfig(t *testing.T) {
+	controller := caddy.NewTestController("dns", "")
+	initialCtx := controller.Context()
+	dnsCtx, ok := initialCtx.(*dnsContext)
+	if !ok {
+		t.Fatalf("controller.Context() did not return a *dnsContext, got %T", initialCtx)
+	}
+	if dnsCtx.keysToConfigs == nil {
+		t.Fatal("dnsCtx.keysToConfigs is nil; it should have been initialized by newContext")
+	}
+
+	t.Run("returns and saves default config when config missing", func(t *testing.T) {
+		controller.ServerBlockIndex = 0
+		controller.ServerBlockKeyIndex = 0
+		key := keyForConfig(controller.ServerBlockIndex, controller.ServerBlockKeyIndex)
+
+		// Ensure config doesn't exist initially for this specific key
+		delete(dnsCtx.keysToConfigs, key)
+
+		cfg := GetConfig(controller)
+		if cfg == nil {
+			t.Fatal("GetConfig returned nil (should create and return a default)")
+		}
+		if len(cfg.ListenHosts) != 1 || cfg.ListenHosts[0] != "" {
+			t.Errorf("Expected default ListenHosts [\"\"] for auto-created config, got %v", cfg.ListenHosts)
+		}
+
+		savedCfg, found := dnsCtx.keysToConfigs[key]
+		if !found {
+			t.Fatal("fallback did not save the default config into the context")
+		}
+		if savedCfg != cfg {
+			t.Fatal("config is not the same instance as the one saved in the context")
+		}
+	})
+}