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/middleware/logger_test.go

@@ -0,0 +1,161 @@
+package middleware
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestResponseWriter_WriteHeader(t *testing.T) {
+	rec := httptest.NewRecorder()
+	rw := &responseWriter{
+		ResponseWriter: rec,
+		status:         http.StatusOK,
+	}
+
+	// First call should set status
+	rw.WriteHeader(http.StatusNotFound)
+	if rw.status != http.StatusNotFound {
+		t.Errorf("status = %d, want %d", rw.status, http.StatusNotFound)
+	}
+	if !rw.wroteHeader {
+		t.Error("wroteHeader should be true after WriteHeader")
+	}
+
+	// Second call should be ignored
+	rw.WriteHeader(http.StatusInternalServerError)
+	if rw.status != http.StatusNotFound {
+		t.Errorf("status = %d, want %d (should not change)", rw.status, http.StatusNotFound)
+	}
+}
+
+func TestResponseWriter_Write(t *testing.T) {
+	rec := httptest.NewRecorder()
+	rw := &responseWriter{
+		ResponseWriter: rec,
+		status:         http.StatusOK,
+	}
+
+	// Write should set default status if not set
+	n, err := rw.Write([]byte("Hello"))
+	if err != nil {
+		t.Errorf("Write() error = %v", err)
+	}
+	if n != 5 {
+		t.Errorf("Write() n = %d, want 5", n)
+	}
+	if rw.written != 5 {
+		t.Errorf("written = %d, want 5", rw.written)
+	}
+	if !rw.wroteHeader {
+		t.Error("wroteHeader should be true after Write")
+	}
+	if rw.status != http.StatusOK {
+		t.Errorf("status = %d, want %d", rw.status, http.StatusOK)
+	}
+
+	// Write more
+	n, err = rw.Write([]byte(" World"))
+	if err != nil {
+		t.Errorf("Write() error = %v", err)
+	}
+	if n != 6 {
+		t.Errorf("Write() n = %d, want 6", n)
+	}
+	if rw.written != 11 {
+		t.Errorf("written = %d, want 11", rw.written)
+	}
+}
+
+func TestResponseWriter_WriteWithExplicitStatus(t *testing.T) {
+	rec := httptest.NewRecorder()
+	rw := &responseWriter{
+		ResponseWriter: rec,
+		status:         http.StatusOK,
+	}
+
+	// Set status first
+	rw.WriteHeader(http.StatusCreated)
+
+	// Write should not change status
+	_, _ = rw.Write([]byte("Created"))
+	if rw.status != http.StatusCreated {
+		t.Errorf("status = %d, want %d", rw.status, http.StatusCreated)
+	}
+}
+
+func TestLogger(t *testing.T) {
+	t.Run("Logs successful request", func(t *testing.T) {
+		handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+			_, _ = w.Write([]byte("OK"))
+		})
+
+		logged := Logger(handler)
+
+		req := httptest.NewRequest(http.MethodGet, "/test", nil)
+		rec := httptest.NewRecorder()
+
+		logged.ServeHTTP(rec, req)
+
+		if rec.Code != http.StatusOK {
+			t.Errorf("Code = %d, want %d", rec.Code, http.StatusOK)
+		}
+		if rec.Body.String() != "OK" {
+			t.Errorf("Body = %q, want %q", rec.Body.String(), "OK")
+		}
+	})
+
+	t.Run("Logs error response", func(t *testing.T) {
+		handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			http.Error(w, "Not Found", http.StatusNotFound)
+		})
+
+		logged := Logger(handler)
+
+		req := httptest.NewRequest(http.MethodGet, "/notfound", nil)
+		rec := httptest.NewRecorder()
+
+		logged.ServeHTTP(rec, req)
+
+		if rec.Code != http.StatusNotFound {
+			t.Errorf("Code = %d, want %d", rec.Code, http.StatusNotFound)
+		}
+	})
+
+	t.Run("Handles POST request", func(t *testing.T) {
+		handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusCreated)
+		})
+
+		logged := Logger(handler)
+
+		req := httptest.NewRequest(http.MethodPost, "/create", nil)
+		rec := httptest.NewRecorder()
+
+		logged.ServeHTTP(rec, req)
+
+		if rec.Code != http.StatusCreated {
+			t.Errorf("Code = %d, want %d", rec.Code, http.StatusCreated)
+		}
+	})
+
+	t.Run("Handles request with no explicit status", func(t *testing.T) {
+		handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			// Just write body without explicit status
+			_, _ = w.Write([]byte("Implicit OK"))
+		})
+
+		logged := Logger(handler)
+
+		req := httptest.NewRequest(http.MethodGet, "/implicit", nil)
+		rec := httptest.NewRecorder()
+
+		logged.ServeHTTP(rec, req)
+
+		// Default status should be 200
+		if rec.Code != http.StatusOK {
+			t.Errorf("Code = %d, want %d", rec.Code, http.StatusOK)
+		}
+	})
+}