feat(types): infer nitro auth endpoint routes

[onmax]

Summary

• add generated Nitro InternalApi augmentation for /api/auth/* endpoint-level inference while keeping runtime route registration as /api/auth/**
• export typed endpoint helpers from #nuxt-better-auth (AuthApiInternalRoutes, AuthApiEndpointPath, AuthApiEndpointMethod, AuthApiEndpointResponse)
• add regression fixture and test for plugin/core endpoint inference (/api/auth/customer/state, /api/auth/get-session)
• update docs examples to rely on inferred endpoint payload types
• include current workspace playground and lockfile updates in this PR as requested

Tests

• pnpm vitest run test/infer-nitro-endpoints-types.test.ts test/infer-plugins-types.test.ts

Closes #194

[pkg-pr-new]

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npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@195

commit: 9de1ebb