fix: skip routes already registered by other plugins

[Tony133]

Proposal:

Registering @fastify/http-proxy on the root together with @fastify/cors fails with:

    Method 'OPTIONS' already declared for route '/*'

because both plugins declare an OPTIONS /* route.

Changes:

• Before registering each proxied route, the plugin now skips the methods already registered for that url (checked with fastify.hasRoute). Preflight requests are then answered by @fastify/cors, and every other request is still proxied.
• The skipped methods are reported with a debug log entry ({ url, methods }), so that an unintended overlap between a local route and the proxy is still visible without turning the legitimate @fastify/cors case into a boot-time warning.
• If no OPTIONS route exists, OPTIONS requests are still proxied upstream as before, so current users see no change.
• README: explain that plugins like @fastify/cors must be registered before the proxy (or OPTIONS removed from httpMethods otherwise), and mention the debug log entry.

Tests:

• New test/existing-routes.js:

  • existing OPTIONS routes are kept and the other methods are proxied
  • OPTIONS is still proxied when no route handles it
  • a route whose methods are all already registered is skipped
  • httpMethods passed as a string
  • prefix alongside a global OPTIONS /* route
  • the skipped methods are logged at debug level with the expected url and methods

The tests register the same routes as @fastify/cors by hand, so no new dev dependency is needed. I also checked the fix manually against @fastify/cors 11.3.0.

Fixes #309

[mcollina]

lgtm