fix(server): don't redirect error responses from #[get] server functions (#5429)

The post-handler redirect logic was unconditionally converting responses
to 302 Found when the request had Accept: text/html and a Referer header.
This overwrote error status codes (4xx, 5xx), making it impossible to
return errors from #[get] endpoints accessed via browser navigation.

Add a response.status().is_success() guard so only successful responses
get the Post/Redirect/Get treatment.

Fixes #5428

packages/fullstack-server/src/serverfn.rs

@@ -113,9 +113,11 @@ impl ServerFunction {
                                 response.headers_mut().extend(headers);
                             }
 
-                            // if it accepts text/html (i.e., is a plain form post) and doesn't already have a
-                            // Location set, then redirect to Referer
-                            if accepts_html {
+                            // If the response is successful and accepts text/html (i.e., is a
+                            // plain form post) and doesn't already have a Location set, then
+                            // redirect to Referer. Only redirect on success so that error
+                            // responses (4xx, 5xx) propagate correctly to the client.
+                            if accepts_html && response.status().is_success() {
                                 if let Some(referrer) = referrer {
                                     let has_location = response.headers().get(LOCATION).is_some();
                                     if !has_location {