test: add comprehensive Go test suite with ~75% coverage

New test files:
- config/config_test.go (100% coverage)
- constants/constants_test.go (100% coverage)
- httputil/response_test.go (100% coverage)
- validation/rules_test.go (91.9% coverage)
- middleware/logger_test.go, security_test.go, security_logger_test.go
- handlers/errors_test.go

Updated documentation:
- doc/27-GO-TESTING.md: Complete testing guide
- doc/00-GO-DOCUMENTATION-INDEX.md: Added testing section
- doc/01-ARCHITECTURE.md: Updated package structure
- doc/DECISIONS.md: Added ADR-004 caching decision
- PROJECT-MEMORY.md: Added Go testing section

internal/constants/constants_test.go

@@ -0,0 +1,148 @@
+package constants
+
+import (
+	"testing"
+)
+
+func TestAllLangs(t *testing.T) {
+	langs := AllLangs()
+
+	if len(langs) != 2 {
+		t.Errorf("Expected 2 languages, got %d", len(langs))
+	}
+
+	// Check that en and es are present
+	hasEn, hasEs := false, false
+	for _, lang := range langs {
+		if lang == LangEnglish {
+			hasEn = true
+		}
+		if lang == LangSpanish {
+			hasEs = true
+		}
+	}
+
+	if !hasEn {
+		t.Error("Expected English (en) to be in AllLangs()")
+	}
+	if !hasEs {
+		t.Error("Expected Spanish (es) to be in AllLangs()")
+	}
+}
+
+func TestIsValidLang(t *testing.T) {
+	tests := []struct {
+		name     string
+		lang     string
+		expected bool
+	}{
+		{"Valid - English", LangEnglish, true},
+		{"Valid - Spanish", LangSpanish, true},
+		{"Invalid - French", "fr", false},
+		{"Invalid - German", "de", false},
+		{"Invalid - Empty", "", false},
+		{"Invalid - Random", "xyz", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := IsValidLang(tt.lang)
+			if result != tt.expected {
+				t.Errorf("IsValidLang(%q) = %v, want %v", tt.lang, result, tt.expected)
+			}
+		})
+	}
+}
+
+func TestValidateLang(t *testing.T) {
+	tests := []struct {
+		name      string
+		lang      string
+		wantError bool
+	}{
+		{"Valid - English", LangEnglish, false},
+		{"Valid - Spanish", LangSpanish, false},
+		{"Invalid - French", "fr", true},
+		{"Invalid - Empty", "", true},
+		{"Invalid - Random", "xyz", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateLang(tt.lang)
+			if (err != nil) != tt.wantError {
+				t.Errorf("ValidateLang(%q) error = %v, wantError %v", tt.lang, err, tt.wantError)
+			}
+		})
+	}
+}
+
+func TestConstants(t *testing.T) {
+	// Test that default language is English
+	if LangDefault != LangEnglish {
+		t.Errorf("LangDefault = %q, want %q", LangDefault, LangEnglish)
+	}
+
+	// Test supported languages map
+	if !SupportedLanguages[LangEnglish] {
+		t.Error("SupportedLanguages should contain English")
+	}
+	if !SupportedLanguages[LangSpanish] {
+		t.Error("SupportedLanguages should contain Spanish")
+	}
+	if SupportedLanguages["fr"] {
+		t.Error("SupportedLanguages should not contain French")
+	}
+}
+
+func TestCVPreferenceConstants(t *testing.T) {
+	// Test CV preference values exist and are non-empty
+	if CVLengthShort == "" {
+		t.Error("CVLengthShort should not be empty")
+	}
+	if CVLengthLong == "" {
+		t.Error("CVLengthLong should not be empty")
+	}
+	if CVIconsShow == "" {
+		t.Error("CVIconsShow should not be empty")
+	}
+	if CVIconsHide == "" {
+		t.Error("CVIconsHide should not be empty")
+	}
+	if CVThemeDefault == "" {
+		t.Error("CVThemeDefault should not be empty")
+	}
+	if CVThemeClean == "" {
+		t.Error("CVThemeClean should not be empty")
+	}
+}
+
+func TestColorThemeConstants(t *testing.T) {
+	if ColorThemeLight == "" {
+		t.Error("ColorThemeLight should not be empty")
+	}
+	if ColorThemeDark == "" {
+		t.Error("ColorThemeDark should not be empty")
+	}
+}
+
+func TestCookieConstants(t *testing.T) {
+	if CookieMaxAge <= 0 {
+		t.Error("CookieMaxAge should be positive")
+	}
+	if CookiePath != "/" {
+		t.Error("CookiePath should be '/'")
+	}
+}
+
+func TestEnvironmentConstants(t *testing.T) {
+	if EnvProduction == "" {
+		t.Error("EnvProduction should not be empty")
+	}
+	if EnvDevelopment == "" {
+		t.Error("EnvDevelopment should not be empty")
+	}
+	if DefaultPort == "" {
+		t.Error("DefaultPort should not be empty")
+	}
+}