feat: add NestJS plugin for type-safe controller method signatures
Generates a `ControllerMethods` type from OpenAPI specs that NestJS developers
can use with `implements Pick<ControllerMethods, ...>` for compile-time contract
enforcement. Mirrors the Fastify plugin pattern with function-type method signatures.
- Plugin files: types.ts, config.ts, index.ts, plugin.ts
- Snapshot tests for 2.0.x, 3.0.x, 3.1.x specs
- Example project with NestJS controller usage
- Documentation and sidebar update
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>