docs(ecosystem): add @guardcore/fastify - #7128

Adds @guardcore/fastify to the Community plugins list in docs/Guides/Ecosystem.md.

It is the Fastify plugin of Guard Core, an application-layer security engine that runs inside the app. It covers request attack detection (XSS, SQL injection, command injection, path traversal), IP and country controls, automatic bans, behavior rules, security headers and per-route decorators. It targets Fastify 5 (peer dependency fastify@^5), it is MIT licensed, and the current version on npm is 4.4.0. The source is in the packages/fastify folder of https://github.com/Guard-Core/guard-core-ts.

The entry sits between @gquittet/graceful-server and @h4ad/serverless-adapter, so the list stays in alphabetical order, and the lines stay within the 80 column limit.

I drafted this with AI assistance. I reviewed the entry myself and I'm opening the PR myself.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct