fix(types): preserve define server auth literals

[onmax]

Summary

• switch defineServerAuth overloads to unconstrained literal inference with ServerAuthConfig validation intersections
• keep runtime behavior unchanged while preserving nested literals like generateId: 'uuid' and additionalFields.*.type: 'string' without as const
• mirror the overload strategy in generated type template declarations
• add regression coverage with positive and negative compile-time assertions for issue 134

Tests

• pnpm lint
• pnpm typecheck
• pnpm vitest run test/define-server-auth-literal-inference.test.ts test/infer-plugins-types.test.ts

Closes #134

[pkg-pr-new]

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npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@135

commit: 539c814