🐛 fix(onboarding): resolve agent route loading stall and branch redirect (#14795)

* 🐛 fix(onboarding): refresh branch config before redirect

* 🐛 fix(onboarding): refresh agent route flag before branch guard

* 🐛 fix(onboarding): simplify agent branch guard

* 🐛 fix(onboarding): eliminate agent route loading stall

- Make AgentModel.getBuiltinAgent idempotent under concurrent callers.
  The web-onboarding builtin agent was inserted by both the bootstrap
  query and the standalone useInitBuiltinAgent SWR in parallel; the
  insert loser hit agents_slug_user_id_unique and SWR sat in its ~5s
  error-retry window before the row could be read.
- Prefetch /onboarding/agent and /onboarding/classic chunks while the
  shared-prefix steps are visible, so the branch redirect no longer
  pays a cold chunk load.

* 🐛 fix(onboarding): skip prefetch under test and complete fixture

- Add `__TEST__` Vite define so renderer code can branch on Vitest runs
  (set true in vitest.config.mts, false in sharedRendererDefine).
- Guard the shared-prefix chunk prefetch with `if (__TEST__) return`.
  Otherwise the fire-and-forget `import('@/routes/onboarding/agent')`
  resolves after the test asserts and tries to load builtin-agents,
  which the test's partial `vi.mock('@lobechat/const')` doesn't supply
  (`DEFAULT_MODEL` missing), surfacing as 25 unhandled rejections.
- Fix `extract.runtime.test.ts` fixture to include the new required
  `agentBenchmarkLoCoMo` field on `MemoryExtractionPrivateConfig`,
  added in 20267fc77c.

vitest.config.mts

@@ -35,6 +35,7 @@ export default defineConfig({
     '__DEV__': process.env.NODE_ENV !== 'production' ? 'true' : 'false',
     '__ELECTRON__': 'false',
     '__MOBILE__': 'false',
+    '__TEST__': 'true',
   },
   optimizeDeps: {
     exclude: ['crypto', 'util', 'tty'],