Fix server functions failing when called from backend code (#4908)

* fix(fullstack-macro): allow server functions to be called without HTTP context

Server functions can now be invoked directly from server-side code without
requiring an HTTP request context. When called outside an HTTP handler,
extractors will use default values instead of failing, matching the behavior
of the old server_fn implementation.

Closes #4907

* fix(fullstack-macro): allow server functions without extractors to be called from backend

Functions with no server extractors use Default::default() and work from backend code.
Functions with extractors still require HTTP context.

Closes #4907

packages/fullstack-macro/src/lib.rs

@@ -432,6 +432,15 @@ fn route_impl_with_route(
 
     let extracted_as_server_headers = route.extracted_as_server_headers(query_tokens.clone());
 
+    // Generate extraction code based on whether there are server extractors
+    let extraction_code = if route.server_args.is_empty() {
+        quote! { Default::default() }
+    } else {
+        quote! {
+            dioxus_fullstack::FullstackContext::extract::<(#(#server_types,)*), _>().await?
+        }
+    };
+
     Ok(quote! {
         #(#fn_docs)*
         #route_docs
@@ -554,8 +563,12 @@ fn route_impl_with_route(
                     )
                 }
 
-                // Extract the server arguments from the context
-                let (#(#server_names,)*) = dioxus_fullstack::FullstackContext::extract::<(#(#server_types,)*), _>().await?;
+                // Extract the server arguments from the context if needed.
+                // If there are no server extractors, use Default::default() which allows
+                // the function to be called from backend code without HTTP context.
+                // If there are server extractors, extraction will fail with a clear error
+                // when called from backend code without HTTP context.
+                let (#(#server_names,)*) = #extraction_code;
 
                 // Call the function directly
                 return #fn_on_server_name #ty_generics(