typed nitro route inference for /api/auth endpoints

[onmax]

Describe the bug

When using @onmax/nuxt-better-auth in a Nuxt 4 app, Nitro route typing for auth endpoints is too generic, so client-side useFetch('/api/auth/customer/state') cannot infer a useful response shape.

Current behavior

Generated Nitro route types contain only a wildcard route:

'/api/auth/**': {
  default: ...ReturnType<typeof import('.../api/auth/[...all]').default>
}

And the catch-all handler type resolves to:

declare const _default: EventHandler<EventHandlerRequest, Promise<Response>>

This makes endpoint-specific inference effectively unavailable for /api/auth/* consumers.

Expected behavior

At least one of the following:

1. Expose endpoint-level typing for known auth routes (e.g. /api/auth/customer/state) so useFetch can infer return types.
2. Export public TS types for key route payloads (customer state, session state, etc.) so consumers can type local wrappers without guessing.
3. Provide a documented helper/composable that returns typed data for these common auth endpoints.

Reproduction

1. Create a Nuxt 4.3.x app with @onmax/nuxt-better-auth.
2. Run nuxi prepare.
3. Inspect .nuxt/types/nitro-routes.d.ts and note '/api/auth/**' wildcard.
4. Use useFetch('/api/auth/customer/state') in app code.
5. Observe that payload shape is not inferred (manual typing is required).

Environment

• Nuxt: 4.3.1
• Nitro: 2.13.1
• @onmax/nuxt-better-auth: 0.0.2-alpha.29

Additional context

This mainly impacts DX and maintainability in app-level composables that consume auth/customer endpoints.