ValidationError: "x-ratelimit-limit" is required

Sentry Issue: [SHIELDS-3H](https://sentry.io/organizations/shields/issues/1138884350/?referrer=github_integration)

```
ValidationError: "x-ratelimit-limit" is required
File "/home/m/shields/services/github/github-api-provider.js", line 81, in GithubApiProvider.getV3RateLimitFromHeaders
const h = Joi.attempt(headers, headerSchema)
File "/home/m/shields/services/github/github-api-provider.js", line 121, in GithubApiProvider.updateToken
} = this.getV3RateLimitFromHeaders(res.headers))
File "/home/m/shields/services/github/github-api-provider.js", line 200, in request
this.updateToken({ token, url, res })
File "/home/m/shields/core/base-service/legacy-request-handler.js", line 223, in Request.request [as _callback]
callback(err, res, body)
...
(5 additional frame(s) were not displayed)
```

Start in services/github/github-api-provider.js at GithubApiProvider.getV3RateLimitFromHeaders and inspect headerSchema, then follow the updateToken and request call paths shown in the report. Reproduce or trace a response missing x-ratelimit-limit, determine the expected handling, and verify that GitHub requests no longer produce this ValidationError.

Written by the indexing model from the issue text.