feat(errors): 统一 AppError（Code/Message/Cause/HTTPCode + Wrap %w + HTTPStatus + ToJSON {error:{code,message}}）

internal/errors/helper.go

@@ -0,0 +1,19 @@
+package errors
+
+import stdErrors "errors"
+
+// IsAppError reports whether err is an *AppError or wraps one anywhere in its
+// chain. It uses errors.As under the hood, so wrapped causes are detected too.
+func IsAppError(err error) bool {
+	var target *AppError
+	return stdErrors.As(err, &target)
+}
+
+// As extracts the first *AppError in err's chain, reporting whether one was
+// found. It mirrors errors.As but returns the typed error directly for
+// convenience. The returned error is the outermost AppError in the chain.
+func As(err error) (*AppError, bool) {
+	var target *AppError
+	ok := stdErrors.As(err, &target)
+	return target, ok
+}