feat: initialize client core and ping endpoint

establishes the foundational architecture for the Onspring Go SDK.

changes included:
- add `Client` struct and `NewClient` constructor with authentication.
- implement functional options pattern (WithHTTPClient, WithBaseURL, etc.).
- add `OnspringAPIError` type for handling non-2xx responses.
- implement internal request construction and execution logic.
- add `Ping` service for API health check verification.
- add comprehensive unit tests and mock server infrastructure.

client_test.go

@@ -0,0 +1,53 @@
+package onspring_test
+
+import (
+	"net/http"
+	"testing"
+	"time"
+
+	"github.com/StevanFreeborn/onspring-api-sdk-go"
+)
+
+func TestNewClient(t *testing.T) {
+	t.Run("it should create client with default settings", func(t *testing.T) {
+		apiKey := "test-api-key"
+		client := onspring.NewClient(apiKey)
+
+		if client.APIKey() != apiKey {
+			t.Errorf("Expected apiKey %s, got %s", apiKey, client.APIKey())
+		}
+
+		if client.BaseURL() != "https://api.onspring.com" {
+			t.Errorf("Expected default baseURL, got %s", client.BaseURL())
+		}
+
+		if client.HTTPClient().Timeout != 120*time.Second {
+			t.Errorf("Expected default timeout, got %v", client.HTTPClient().Timeout)
+		}
+
+	})
+
+	t.Run("it should create client with custom settings", func(t *testing.T) {
+		apiKey := "test-api-key"
+		customURL := "https://custom.onspring.com"
+		customHTTPClient := &http.Client{Timeout: 50 * time.Second}
+
+		client := onspring.NewClient(
+			apiKey,
+			onspring.WithBaseURL(customURL),
+			onspring.WithHTTPClient(customHTTPClient),
+		)
+
+		if client.APIKey() != apiKey {
+			t.Errorf("Expected apiKey %s, got %s", apiKey, client.APIKey())
+		}
+
+		if client.BaseURL() != customURL {
+			t.Errorf("Expected baseURL %s, got %s", customURL, client.BaseURL())
+		}
+
+		if client.HTTPClient() != customHTTPClient {
+			t.Errorf("Expected custom HTTP client, got %v", client.HTTPClient())
+		}
+	})
+}