fix(rp-connect): clamp onboarding topic RF to broker count

The RPCN onboarding Add Topic step spread TOPIC_FORM_DEFAULTS
(replicationFactor: 3) into the form and renders the RF field as
readOnly in AdvancedTopicSettings. On single-broker clusters
(local-byoc dev environments) that meant the user saw RF=3 with no
way to edit, and CreateTopic failed with "not enough replicas".

Override replicationFactor at form init with min(default, brokersOnline)
via useGetKafkaInfoQuery so the readOnly value matches what the broker
can actually satisfy.

Author: c-julin

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

@@ -25,6 +25,7 @@ import { ListTopicsRequestSchema } from 'protogen/redpanda/api/dataplane/v1/topi
 import { listTopics } from 'protogen/redpanda/api/dataplane/v1/topic-TopicService_connectquery';
 import { forwardRef, useCallback, useEffect, useImperativeHandle, useMemo, useState } from 'react';
 import { useForm, useWatch } from 'react-hook-form';
+import { useGetKafkaInfoQuery } from 'react-query/api/cluster-status';
 import { useLegacyListTopicsQuery } from 'react-query/api/topic';
 import { LONG_LIVED_CACHE_STALE_TIME } from 'react-query/react-query.utils';
 import { isFalsy } from 'utils/falsy';
@@ -104,12 +105,23 @@ export const AddTopicStep = forwardRef<BaseStepRef<AddTopicFormData>, AddTopicSt
 
     const isPending = createTopicMutation.isPending;
 
+    // The RF field is readOnly in advanced-topic-settings, so the default is
+    // also the final value. Clamp to broker count so single-broker clusters
+    // (e.g. local-byoc) don't hit "not enough replicas" on CreateTopic.
+    const { data: kafkaInfo } = useGetKafkaInfoQuery();
+    const brokersOnline = kafkaInfo?.brokersOnline ?? 0;
+    const defaultReplicationFactor =
+      brokersOnline > 0
+        ? Math.min(TOPIC_FORM_DEFAULTS.replicationFactor, brokersOnline)
+        : TOPIC_FORM_DEFAULTS.replicationFactor;
+
     const defaultValues = useMemo(
       () => ({
         ...TOPIC_FORM_DEFAULTS,
+        replicationFactor: defaultReplicationFactor,
         topicName: defaultTopicName || TOPIC_FORM_DEFAULTS.topicName,
       }),
-      [defaultTopicName]
+      [defaultTopicName, defaultReplicationFactor]
     );
 
     const form = useForm<AddTopicFormData>({