fix(topics): clamp default replication factor to broker count (#2420)

* fix(topics): clamp default replication factor to broker count

CreateTopic defaulted replicationFactor to 3 in both the topic-create
modal and the MCP inspector's "create new topic" flow. On single-broker
clusters (local-byoc dev environments, etc.) that rejected the request
with "not enough replicas". Clamp the default to min(3, brokersOnline)
and fall back to 3 while KafkaInfo is loading.

* 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.

* revert(mcp): drop RF clamp from MCP inspector

Scope back to the user-visible topic-create paths only. The MCP
inspector's "create new topic" flow is out of scope for this PR.

* fix(rp-connect): react to late-arriving KafkaInfo via rhf `values`

useForm captures defaultValues at mount; when the KafkaInfo query
resolves after mount the RF field stays at its initial 3. Pass
defaultValues through both `defaultValues` and `values` with
`resetOptions: { keepDirtyValues: true }` so the form reactively
picks up the clamped replicationFactor once brokersOnline is known,
without clobbering fields the user has already edited.

* 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

@@ -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';
@@ -104,18 +105,36 @@ 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]
     );
 
+    // Pass defaultValues AND values so react-hook-form reactively updates
+    // replicationFactor when the KafkaInfo query resolves after mount. The
+    // `values` prop is rhf's built-in mechanism for external reactive state;
+    // `keepDirtyValues` prevents fields the user has already touched from
+    // being overwritten when kafkaInfo lands late.
     const form = useForm<AddTopicFormData>({
       resolver: zodResolver(addTopicFormSchema),
       mode: 'onChange',
       defaultValues,
+      values: defaultValues,
+      resetOptions: { keepDirtyValues: true },
     });
 
     const watchedTopicName = useWatch({
@@ -147,7 +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

@@ -20,6 +20,7 @@ import {
   CreateTopicRequestSchema,
   ListTopicsRequestSchema,
 } from 'protogen/redpanda/api/dataplane/v1/topic_pb';
+import { useGetKafkaInfoQuery } from 'react-query/api/cluster-status';
 import { useCreateTopicMutation, useLegacyListTopicsQuery } from 'react-query/api/topic';
 import { z } from 'zod';
 
@@ -44,6 +45,14 @@ export const CreateTopicModal = ({ isOpen, onClose }: CreateTopicModalProps) =>
     hideInternalTopics: true,
   });
   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". 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 =
+    brokersOnline > 0 ? Math.min(DEFAULT_TOPIC_REPLICATION_FACTOR, brokersOnline) : DEFAULT_TOPIC_REPLICATION_FACTOR;
 
   const formOpts = formOptions({
     defaultValues: {
@@ -57,7 +66,7 @@ export const CreateTopicModal = ({ isOpen, onClose }: CreateTopicModalProps) =>
         topic: create(CreateTopicRequest_TopicSchema, {
           name: value.name,
           partitionCount: DEFAULT_TOPIC_PARTITION_COUNT,
-          replicationFactor: DEFAULT_TOPIC_REPLICATION_FACTOR,
+          replicationFactor,
           configs: [
             create(CreateTopicRequest_Topic_ConfigSchema, {
               name: 'cleanup.policy',