types: restrict route lookup methods (#6963)

#### Description

`hasRoute` and `findRoute` look up a route for a single HTTP method, but
their TypeScript definitions reused `RouteOptions.method`, which also
allows method arrays for route registration.

Both APIs forward one method to the router after normalizing it with
`toUpperCase()`. Arrays are only meaningful during route registration;
for a lookup they have no defined result semantics and currently cause a
runtime `TypeError`.

This narrows the `method` option for both lookup APIs to `HTTPMethods`
and adds type tests to reject method arrays.
#### Checklist

- [ ] run `npm run test && npm run benchmark --if-present`
- [x] tests and/or benchmarks are included
- [ ] documentation is changed or added
- [x] commit message and code follows the [Developer's Certificate of
Origin](https://github.com/fastify/.github/blob/main/CONTRIBUTING.md#developers-certificate-of-origin-11)
and the [Code of
conduct](https://github.com/fastify/.github/blob/main/CODE_OF_CONDUCT.md)

Signed-off-by: haoxiang Yan <108142374+Yanhaoxi@users.noreply.github.com>