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.

ping_test.go

@@ -0,0 +1,103 @@
+package onspring_test
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"github.com/StevanFreeborn/onspring-api-sdk-go"
+)
+
+func TestGet(t *testing.T) {
+	t.Run("it should return an error if context is nil", func(t *testing.T) {
+		_, client := setupMockServer(t, func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		})
+
+		err := client.Ping.Get(nil)
+
+		if err == nil {
+			t.Errorf("Expected error for nil context, got nil")
+		}
+	})
+
+	t.Run("it should return an error if context is canceled", func(t *testing.T) {
+		_, client := setupMockServer(t, func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		})
+
+		ctx, cancel := context.WithCancel(context.Background())
+
+		cancel()
+
+		err := client.Ping.Get(ctx)
+
+		if err == nil {
+			t.Errorf("Expected error for canceled context, got nil")
+		}
+	})
+
+	t.Run("it should return an error if encounters a network error", func(t *testing.T) {
+		client := onspring.NewClient(
+			"test-api-key",
+			onspring.WithBaseURL("http://invalid-url"),
+			onspring.WithHTTPClient(&http.Client{Transport: &ErrorTransport{}}),
+		)
+
+		err := client.Ping.Get(context.Background())
+
+		if err == nil {
+			t.Errorf("Expected network error, got nil")
+		}
+	})
+
+	t.Run("it should return an error if create a request fails", func(t *testing.T) {
+		_, client := setupMockServer(t, func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		})
+
+		invalidClient := onspring.NewClient(
+			"test-api-key",
+			onspring.WithBaseURL("http://[::1]:namedport"),
+			onspring.WithHTTPClient(client.HTTPClient()),
+		)
+
+		err := invalidClient.Ping.Get(context.Background())
+
+		if err == nil {
+			t.Errorf("Expected request creation error, got nil")
+		}
+	})
+
+	t.Run("it should perform a GET request to the /ping endpoint and return no error if receives 200 status code", func(t *testing.T) {
+		_, client := setupMockServer(t, func(w http.ResponseWriter, r *http.Request) {
+			if r.Method != http.MethodGet {
+				t.Errorf("Expected GET method, got %s", r.Method)
+			}
+
+			if r.URL.Path != "/ping" {
+				t.Errorf("Expected /ping endpoint, got %s", r.URL.Path)
+			}
+
+			w.WriteHeader(http.StatusOK)
+		})
+
+		err := client.Ping.Get(context.Background())
+
+		if err != nil {
+			t.Errorf("Expected no error, got %v", err)
+		}
+	})
+
+	t.Run("it should return an error if the /ping endpoint returns a non-200 status code", func(t *testing.T) {
+		_, client := setupMockServer(t, func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusInternalServerError)
+		})
+
+		err := client.Ping.Get(context.Background())
+
+		if err == nil {
+			t.Errorf("Expected error, got nil")
+		}
+	})
+}