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.

error_test.go

@@ -0,0 +1,40 @@
+package onspring_test
+
+import (
+	"testing"
+
+	"github.com/StevanFreeborn/onspring-api-sdk-go"
+)
+
+func TestOnspringAPIError(t *testing.T) {
+	t.Run("it should create OnspringAPIError instance", func(t *testing.T) {
+		statusCode := 500
+		message := "Internal Server Error"
+
+		err := &onspring.OnspringAPIError{
+			StatusCode: statusCode,
+			Message:    message,
+		}
+
+		if err.StatusCode != statusCode {
+			t.Errorf("Expected status code %d, got %d", statusCode, err.StatusCode)
+		}
+
+		if err.Message != message {
+			t.Errorf("Expected message %s, got %s", message, err.Message)
+		}
+	})
+
+	t.Run("it should return formatted error message", func(t *testing.T) {
+		err := &onspring.OnspringAPIError{
+			StatusCode: 404,
+			Message:    "Not Found",
+		}
+
+		expectedMessage := "onspring api error: status=404 message=Not Found"
+
+		if err.Error() != expectedMessage {
+			t.Errorf("Expected error message %s, got %s", expectedMessage, err.Error())
+		}
+	})
+}