fix: Handle HTTP errors in FromResponseParts (#5243)

* fix: Handle HTTP errors in FromResponseParts

Check for non-success status codes before attempting to decode response
parts. For error responses, parse the JSON error payload and convert it
to a ServerFnError::ServerError.

Also make ErrorPayload fields pub(crate) to allow internal access.

* fixed test name

packages/fullstack/src/magic.rs

@@ -87,12 +87,12 @@ pub enum RestEndpointPayload<T, E> {
 /// The error payload structure for REST API errors.
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
 pub struct ErrorPayload<E> {
-    message: String,
+    pub(crate) message: String,
 
-    code: u16,
+    pub(crate) code: u16,
 
     #[serde(skip_serializing_if = "Option::is_none")]
-    data: Option<E>,
+    pub(crate) data: Option<E>,
 }
 
 /// Convert a `RequestError` into a `ServerFnError`.