runtimeConfig schema validation

Describe the feature

Instead of defining overridable empty values inside the (nitro|nuxt).config.ts file, we should be able to define a schema using a validation library like zod or valibot.

Actual overridable values runtimeConfig

//https://nitro.unjs.io/config
export default defineNitroConfig({
  srcDir: "server",

  runtimeConfig: {
    betterAuth: {
      secret: "",
      url: "",
    },
    turnstile: {
      secretKey: "",
    },
    cronSecret: "",
    upstash: {
      redis: {
        url: "",
        token: "",
      },
    },
    posthog: {
      apiKey: "",
      apiHost: "",
    },
  },
});

Schema based runtimeConfig

import { z } from "zod";

//https://nitro.unjs.io/config
export default defineNitroConfig({
  srcDir: "server",

  runtimeConfig: {
    betterAuth: {
      secret: z.string().min(32).max(32),
      url: z.string().url(),
    },
    turnstile: {
      secretKey: z.string().min(1),
    },
    cronSecret: z.string().min(1),
    upstash: {
      redis: {
        url: z.string().min(1),
        token: z.string().min(1),
      },
    },
    posthog: {
      apiKey: z.string().min(1),
      apiHost: z.string().min(1),
    },
  },
});

Additional information

• Would you be willing to help implement this feature?

Start with the (nitro|nuxt).config.ts runtimeConfig examples and the Nitro configuration documentation linked in the issue. Define how schema values from libraries such as zod or valibot should be accepted and when runtime values are validated; done means the documented configuration supports the shown schema-based form with clear validation behavior.