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.go

@@ -0,0 +1,24 @@
+package onspring
+
+import (
+	"fmt"
+)
+
+// OnspringAPIError represents an error returned by the Onspring API.
+// It contains the HTTP status code and the error message from the API response.
+// This error type implements the error interface.
+type OnspringAPIError struct {
+	// StatusCode is the HTTP status code returned by the API.
+	StatusCode int
+	// Message is the error message returned by the API or the HTTP status text.
+	Message string
+}
+
+// Error returns a formatted error string containing the status code and message.
+// It implements the error interface for OnspringAPIError.
+//
+// Returns:
+//   - string: A formatted error message in the format "onspring api error: status={code} message={message}"
+func (e *OnspringAPIError) Error() string {
+	return fmt.Sprintf("onspring api error: status=%d message=%s", e.StatusCode, e.Message)
+}