feat: gate node replacement loading on server feature flag (#8750)

## Summary

Gates the node replacement store's `load()` call behind the
`node_replacements` server feature flag, so the frontend only calls
`/api/node_replacements` when the backend advertises support.

## Changes

- Added `NODE_REPLACEMENTS = 'node_replacements'` to `ServerFeatureFlag`
enum
- Added `nodeReplacementsEnabled` getter to `useFeatureFlags()`
- Added `api.serverSupportsFeature('node_replacements')` guard in
`useNodeReplacementStore.load()`

## Context

Without this guard, the frontend would attempt to fetch node
replacements from backends that don't support the endpoint, causing 404
errors.

Companion backend PR: https://github.com/Comfy-Org/ComfyUI/pull/12362

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8750-feat-gate-node-replacement-loading-on-server-feature-flag-3026d73d365081ec9246d77ad88f5bdc)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Jin Yi <jin12cc@gmail.com>
Co-authored-by: Alexander Brown <drjkl@comfy.org>
Co-authored-by: Claude <noreply@anthropic.com>

src/platform/nodeReplacement/nodeReplacementStore.test.ts

@@ -15,6 +15,18 @@ vi.mock('./nodeReplacementService', () => ({
   fetchNodeReplacements: vi.fn()
 }))
 
+const mockNodeReplacementsEnabled = vi.hoisted(() => ({ value: true }))
+
+vi.mock('@/composables/useFeatureFlags', () => ({
+  useFeatureFlags: vi.fn(() => ({
+    flags: {
+      get nodeReplacementsEnabled() {
+        return mockNodeReplacementsEnabled.value
+      }
+    }
+  }))
+}))
+
 function mockSettingStore(enabled: boolean) {
   vi.mocked(useSettingStore, { partial: true }).mockReturnValue({
     get: vi.fn().mockImplementation((key: string) => {
@@ -27,9 +39,10 @@ function mockSettingStore(enabled: boolean) {
   })
 }
 
-function createStore(enabled = true) {
+function createStore(enabled = true, featureEnabled = true) {
   setActivePinia(createPinia())
   mockSettingStore(enabled)
+  mockNodeReplacementsEnabled.value = featureEnabled
   return useNodeReplacementStore()
 }
 
@@ -38,6 +51,7 @@ describe('useNodeReplacementStore', () => {
 
   beforeEach(() => {
     vi.clearAllMocks()
+    mockNodeReplacementsEnabled.value = true
     store = createStore(true)
   })
 
@@ -257,5 +271,15 @@ describe('useNodeReplacementStore', () => {
       expect(fetchNodeReplacements).not.toHaveBeenCalled()
       expect(store.isLoaded).toBe(false)
     })
+
+    it('should not call API when server feature flag is disabled', async () => {
+      vi.mocked(fetchNodeReplacements).mockResolvedValue(mockReplacements)
+      store = createStore(true, false)
+
+      await store.load()
+
+      expect(fetchNodeReplacements).not.toHaveBeenCalled()
+      expect(store.isLoaded).toBe(false)
+    })
   })
 })

src/platform/nodeReplacement/nodeReplacementStore.ts

@@ -3,6 +3,7 @@ import type { NodeReplacement, NodeReplacementResponse } from './types'
 import { defineStore } from 'pinia'
 import { computed, ref } from 'vue'
 
+import { useFeatureFlags } from '@/composables/useFeatureFlags'
 import { useSettingStore } from '@/platform/settings/settingStore'
 import { fetchNodeReplacements } from './nodeReplacementService'
 
@@ -14,8 +15,12 @@ export const useNodeReplacementStore = defineStore('nodeReplacement', () => {
     settingStore.get('Comfy.NodeReplacement.Enabled')
   )
 
+  const { flags } = useFeatureFlags()
+
   async function load() {
     if (!isEnabled.value || isLoaded.value) return
+    if (!flags.nodeReplacementsEnabled) return
+
     try {
       replacements.value = await fetchNodeReplacements()
       isLoaded.value = true