chore(index): add jsdoc types

[Fdawgs]

This PR adds JSDoc blocks to all functions to improve static analysis. We had some success with this in fastify/secure-json-parse#154 so seeing if it shakes anything out in other repos.

Checklist

• run npm run test && npm run benchmark --if-present
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Copilot]

Pull request overview

This PR adds JSDoc type annotations to improve static analysis capabilities across the codebase, including @this annotations for context-dependent functions and @type annotations for better type inference.

• Adds JSDoc type annotations to main plugin functions in index.js
• Adds TypeScript type hints to test functions using TestContext from node:test

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
index.js	Adds @this annotations for acceptsMethod and replyAcceptMethod, and @type annotation for the main fastifyAccepts function
test/index.test.js	Adds TestContext typedef and inline type annotations to test callback parameters

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

You can also share your feedback on Copilot code review for a chance to win a $100 gift card. Take the survey.