Add validation and error handling to NewClient

- NewClient now returns (*Client, error) instead of *Client
- Validate that BaseURL, TokenID, and TokenSecret are non-empty
- Strip trailing slash from BaseURL
- Add comprehensive unit tests for NewClient

bookstack_test.go

@@ -0,0 +1,94 @@
+package bookstack
+
+import (
+	"net/http"
+	"testing"
+)
+
+func TestNewClient(t *testing.T) {
+	t.Run("success with all fields", func(t *testing.T) {
+		c, err := NewClient(Config{
+			BaseURL:     "https://docs.example.com",
+			TokenID:     "abc",
+			TokenSecret: "xyz",
+		})
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if c.baseURL != "https://docs.example.com" {
+			t.Errorf("baseURL = %q, want %q", c.baseURL, "https://docs.example.com")
+		}
+	})
+
+	t.Run("default HTTPClient", func(t *testing.T) {
+		c, err := NewClient(Config{
+			BaseURL:     "https://docs.example.com",
+			TokenID:     "abc",
+			TokenSecret: "xyz",
+		})
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if c.httpClient == nil {
+			t.Fatal("httpClient should not be nil")
+		}
+	})
+
+	t.Run("custom HTTPClient", func(t *testing.T) {
+		custom := &http.Client{}
+		c, err := NewClient(Config{
+			BaseURL:     "https://docs.example.com",
+			TokenID:     "abc",
+			TokenSecret: "xyz",
+			HTTPClient:  custom,
+		})
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if c.httpClient != custom {
+			t.Error("expected custom HTTPClient to be used")
+		}
+	})
+
+	t.Run("trailing slash stripped", func(t *testing.T) {
+		c, err := NewClient(Config{
+			BaseURL:     "https://docs.example.com/",
+			TokenID:     "abc",
+			TokenSecret: "xyz",
+		})
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if c.baseURL != "https://docs.example.com" {
+			t.Errorf("baseURL = %q, want trailing slash stripped", c.baseURL)
+		}
+	})
+
+	t.Run("error on missing BaseURL", func(t *testing.T) {
+		_, err := NewClient(Config{TokenID: "abc", TokenSecret: "xyz"})
+		if err == nil {
+			t.Fatal("expected error for missing BaseURL")
+		}
+	})
+
+	t.Run("error on missing TokenID", func(t *testing.T) {
+		_, err := NewClient(Config{BaseURL: "https://x.com", TokenSecret: "xyz"})
+		if err == nil {
+			t.Fatal("expected error for missing TokenID")
+		}
+	})
+
+	t.Run("error on missing TokenSecret", func(t *testing.T) {
+		_, err := NewClient(Config{BaseURL: "https://x.com", TokenID: "abc"})
+		if err == nil {
+			t.Fatal("expected error for missing TokenSecret")
+		}
+	})
+
+	t.Run("error on all fields missing", func(t *testing.T) {
+		_, err := NewClient(Config{})
+		if err == nil {
+			t.Fatal("expected error for all missing fields")
+		}
+	})
+}