types!: align cors with registration-scoped typing model

[mcollina]

Summary

Update @fastify/cors typing/tests/docs for the registration-scoped typing
migration.

@fastify/cors is the control case in this stack (no decorator widening), so
this PR ensures registration keeps instance shape stable while still aligning
with the new core model.

Changes include:

• Type tests asserting instance shape is preserved after registration;
• README TypeScript guidance aligned with registration-scoped typing.

Dependency

This PR depends on the Fastify core typing migration:

• types!: remove declaration-merging reliance with registration-scoped decorator typing fastify#6581 (feat/typed-decorators)

Validation

• npm run test:typescript ✅
• npm run lint -- types ✅

Context

Refs fastify/fastify#5061.