Query and Mutate surface raw HTTP errors instead of *HTTPError #293

Bug

GraphQL calls through GraphQLClient.Query/Mutate turn non-2xx HTTP responses into a raw, unstructured error:

non-200 OK status code: 401 Unauthorized body: "{\"message\":\"Bad credentials\",...}"

This is visible to users as, e.g., gh pr status / gh gist list / gh release list failing with that raw string when the token is invalid (see cli/cli#8434). The caller cannot parse the status code, headers, or body, because the underlying GraphQL client (cli/shurcooL-graphql) formats the whole response into a plain fmt.Errorf string.

The Do/DoWithContext methods handle this properly via HandleHTTPError (returning *HTTPError); only the named QueryNamed/MutateNamed path leaks the raw error.

Proposed approach

• Wrap the inner GraphQL HTTP client's transport so that responses outside the 2xx range become an *HTTPError (existing HandleHTTPError), so the underlying GraphQL client never formats the raw response.
• In QueryWithContext/MutateWithContext, unwrap the *url.Error that net/http adds around round-trip failures and return the bare *HTTPError, consistent with the Do path.

After the change, an unauthenticated gh pr status shows:

HTTP 401: Bad credentials (https://api.github.com/graphql)
Try authenticating with: gh auth login

This matches the approach suggested by @williammartin in cli/cli#8434 ("capture this error in go-gh and wrap correctly").

I have an implementation ready with tests (query/mutate/do HTTP-error cases) that I'm happy to submit as a PR if you give the go-ahead.

Implementation note: prepared with assistance from an AI coding tool; the change and its tests were manually verified against the repository.