fix(topics): isolate keepDirtyValues to RPCN form watch only

Address PR review feedback on #2420. The form-level
`resetOptions: { keepDirtyValues: true }` (added so dirty fields
survive the late `kafkaInfo` re-init) was leaking into the
existing-topic-selected reset, where dirty fields should be
overwritten by the selected topic's actual config.

Also tighten the RF clamp comment in `create-topic-modal.tsx` to
cover the all-brokers-offline case, not just the loading case.

Refs UX-1208.

Author: c-julin

frontend/src/components/pages/rp-connect/onboarding/add-topic-step.tsx

@@ -166,7 +166,10 @@ export const AddTopicStep = forwardRef<BaseStepRef<AddTopicFormData>, AddTopicSt
       }
       if (topicConfig && !topicConfig.error) {
         const allTopicValues = parseTopicConfigFromExisting(existingTopicSelected, topicConfig);
-        form.reset(allTopicValues, { keepDefaultValues: false });
+        // Override the form-level `keepDirtyValues: true` default — when a user
+        // selects an existing topic, its config must fully replace any partial
+        // input they've made.
+        form.reset(allTopicValues, { keepDefaultValues: false, keepDirtyValues: false });
       } else {
         form.setValue('topicName', existingTopicSelected.topicName, {
           shouldDirty: false,

frontend/src/components/pages/topics/create-topic-modal.tsx

@@ -46,8 +46,9 @@ export const CreateTopicModal = ({ isOpen, onClose }: CreateTopicModalProps) =>
   });
   const { mutateAsync: createTopic, isPending: isCreateTopicPending } = useCreateTopicMutation();
   // Clamp RF to broker count so single-broker clusters (e.g. local-byoc) don't
-  // fail CreateTopic with "not enough replicas". Fall back to the default if
-  // the KafkaInfo query hasn't resolved yet.
+  // fail CreateTopic with "not enough replicas". When `brokersOnline` is 0
+  // (KafkaInfo still loading, or every broker is offline) keep the default —
+  // a degraded cluster will reject CreateTopic regardless of what we send.
   const { data: kafkaInfo } = useGetKafkaInfoQuery();
   const brokersOnline = kafkaInfo?.brokersOnline ?? 0;
   const replicationFactor =