fix: conditionally import param and requestBody in controller template

[mmustafasenoglu]

Summary

Fixes #3335 - The OpenAPI controller template unconditionally imported param and requestBody decorators from @loopback/rest, even when the generated controller did not use any parameters or request bodies.

Changes

• spec-helper.js: Added tracking for whether @param and @requestBody decorators are used in controller methods
• controller-template.ts.ejs: Conditionally import these decorators only when needed

Before

import {api, operation, param, requestBody} from "@loopback/rest";

export class MyController {
  constructor() {}
  
  async getAll(): Promise<string[]> {
    throw new Error("Not implemented");
  }
}

After

import {api, operation} from "@loopback/rest";

export class MyController {
  constructor() {}
  
  async getAll(): Promise<string[]> {
    throw new Error("Not implemented");
  }
}

Benefits

• Eliminates ESLint no-unused-imports errors
• Cleaner generated code
• Better developer experience

Closes #3335