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.

infra_test.go

@@ -0,0 +1,34 @@
+package onspring_test
+
+import (
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/StevanFreeborn/onspring-api-sdk-go"
+)
+
+func setupMockServer(t *testing.T, handler http.HandlerFunc) (*httptest.Server, *onspring.Client) {
+	t.Helper()
+
+	server := httptest.NewServer(handler)
+
+	t.Cleanup(func() {
+		server.Close()
+	})
+
+	client := onspring.NewClient(
+		"test-key",
+		onspring.WithBaseURL(server.URL),
+		onspring.WithHTTPClient(server.Client()),
+	)
+
+	return server, client
+}
+
+type ErrorTransport struct{}
+
+func (t *ErrorTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	return nil, errors.New("simulated network connection failure")
+}