test(middleware): add unit tests for middleware behavior

- remove redundant check on registry.Update and check if slug exist before locking the mutex
- Update SonarQube action to not use Go cache when setting up Go

internal/middleware/tunnelfingerprint_test.go

@@ -0,0 +1,76 @@
+package middleware
+
+import (
+	"errors"
+	"testing"
+)
+
+type mockResponseHeader struct {
+	headers map[string]string
+}
+
+func (m *mockResponseHeader) Value(key string) string {
+	return m.headers[key]
+}
+
+func (m *mockResponseHeader) Set(key string, value string) {
+	m.headers[key] = value
+}
+
+func (m *mockResponseHeader) Remove(key string) {
+	delete(m.headers, key)
+}
+
+func (m *mockResponseHeader) Finalize() []byte {
+	return nil
+}
+
+func TestTunnelFingerprintHandleResponse(t *testing.T) {
+	tests := []struct {
+		name         string
+		initialState map[string]string
+		expected     map[string]string
+		body         []byte
+		wantErr      error
+	}{
+		{
+			name:         "Sets Server Header",
+			initialState: map[string]string{},
+			expected:     map[string]string{"Server": "Tunnel Please"},
+			body:         []byte("Sample body"),
+			wantErr:      nil,
+		},
+		{
+			name:         "Overwrites Server Header",
+			initialState: map[string]string{"Server": "Old Value"},
+			expected:     map[string]string{"Server": "Tunnel Please"},
+			body:         nil,
+			wantErr:      nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			mockHeader := &mockResponseHeader{headers: tt.initialState}
+			tunnelFingerprint := NewTunnelFingerprint()
+
+			err := tunnelFingerprint.HandleResponse(mockHeader, tt.body)
+			if !errors.Is(err, tt.wantErr) {
+				t.Fatalf("unexpected error, got: %v, want: %v", err, tt.wantErr)
+			}
+
+			for key, expectedValue := range tt.expected {
+				if val := mockHeader.Value(key); val != expectedValue {
+					t.Errorf("header[%q] = %q; want %q", key, val, expectedValue)
+				}
+			}
+		})
+	}
+}
+
+func TestNewTunnelFingerprint(t *testing.T) {
+	instance := NewTunnelFingerprint()
+	if instance == nil {
+		t.Errorf("NewTunnelFingerprint() = nil; want non-nil instance")
+	}
+}