fix: skip redirect and error fallback on middleware mode

[antfu]

Description

When serving vite server as middleware mode with base, it can't be used with sub routes as the base middleware will always redirect / to the base.

Consider base is set to /admin/ in the following examples.

// the default handle is been override with vite's fallback
// a.k.a only vite server work, but not the server original handlers
app.use(server.middlewares)

// the original handlers works, but as the base in `req.url` passed to vite is striped off by `connect`
// which cause the middleware resolve to `/admin/admin/` 
app.use('/admin/', server.middlewares)

Neither of these usages work. I think the baseMiddware is more like a friendly guide and warning for standalone vite server but not actually for middleware mode. Skipped the middleware in this PR.

Additional context

Current workaround:

  const middlewares = server.middlewares

  // remove Vite's base middleware since it's already handled by connect route
  // it's right before '/__open-in-editor' middleware
  const viteBaseMiddlewareIndex = middlewares.stack.findIndex(i => i.route === '/__open-in-editor') - 1
  if (viteBaseMiddlewareIndex >= 0) {
    middlewares.stack.splice(viteBaseMiddlewareIndex, 1)
  }

  app.use('/admin/', middlewares)

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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