Use custom error code when rejecting requests in Funrun (#23542)

This will allow us to retry actions that were immediately rejected due to server overload safely.

GitOrigin-RevId: 2636064e7f1ffcc24d91f24992bdd03095bc07cc

crates/function_runner/src/server.rs

@@ -588,7 +588,7 @@ mod tests {
         let response =
             FunctionRunnerCore::<TestRuntime, Arc<dyn Storage>>::receive_response(rx2).await?;
         let err = response.unwrap_err();
-        assert!(err.is_overloaded());
+        assert!(err.is_rejected_before_execution());
         assert!(err.to_string().contains(NO_AVAILABLE_WORKERS));
         Ok(())
     }
@@ -639,7 +639,7 @@ mod tests {
         let response =
             FunctionRunnerCore::<TestRuntime, Arc<dyn Storage>>::receive_response(rx2).await?;
         let err = response.unwrap_err();
-        assert!(err.is_overloaded());
+        assert!(err.is_rejected_before_execution());
         assert!(err.to_string().contains(NO_AVAILABLE_WORKERS));
         Ok(())
     }

crates/isolate/src/client.rs

@@ -451,9 +451,9 @@ impl<RT: Runtime> Request<RT> {
     }
 
     fn reject(self) {
-        // TODO: Create a concrete error type for this so FunRun knows when to respond
-        // with a gRPC RESOURCE_EXHAUSTED code.
-        let error = ErrorMetadata::overloaded("WorkerOverloaded", NO_AVAILABLE_WORKERS).into();
+        let error =
+            ErrorMetadata::rejected_before_execution("WorkerOverloaded", NO_AVAILABLE_WORKERS)
+                .into();
         match self.inner {
             RequestType::Udf { response, .. } => {
                 let _ = response.send(Err(error));