Invalid calls to POST …/scale return JSON parse errors

While reviewing #8207, I thought I’d manipulate the calls to `POST /v1/job/:job/scale` to see what would happen if I passed invalid `Count` values.

With `"Count":1.5` the response was:

```
json: cannot unmarshal number 1.5 into Go struct field ScalingRequest.Count of type int64
```

With `"Count":"xyz"` the response was:

```
json: cannot unmarshal string into Go struct field ScalingRequest.Count of type int64
```

Both of these responses came with HTTP status 400.

Example cURL

curl 'http://localhost:4300/v1/job/api/scale' \                                  

  -H 'content-type: application/json; charset=UTF-8' \

  --data-binary '{"Count":"xyz","Reason":"Manual scaling event from the Nomad UI","Target":{"Group":"endpoints"},"Meta":{"Source":"nomad-ui"}}' \

It would be nice from an API consumer perspective for these responses to come as JSON, perhaps status 422?

Start at the POST /v1/job/:job/scale handler and the ScalingRequest.Count decoding path. Reproduce the requests with fractional and string Count values, then inspect nearby API tests if available. Done means invalid Count inputs return a JSON error response with an agreed client-error status, rather than raw JSON parse errors.

Written by the indexing model from the issue text.