refactor(errors): explain failed requests through the SDK

The hints for a missing repository, a pull request number that names an
issue and an existing pull request read the API with raw GETs from the app
layer. Read them through SDK lookups in services, which reuse the failed
request's instance and credential, and let transport only hand out that
client.

Signed-off-by: bircni <bircni@icloud.com>

internal/transport/client_test.go

@@ -232,12 +232,11 @@ func TestAPIErrorLookupRepeatsReadsWithTheFailedCredential(t *testing.T) {
 	var apiErr *APIError
 	require.ErrorAs(t, err, &apiErr)
 	require.Equal(t, server.URL+"/base", apiErr.Instance)
-	var repo struct {
-		FullName  string `json:"full_name"`
-		HasIssues bool   `json:"has_issues"`
-	}
-	require.Equal(t, http.StatusOK, apiErr.Lookup(context.Background(), "repos/a/b", &repo))
-	require.Equal(t, "a/b", repo.FullName)
-	require.Equal(t, http.StatusNotFound, apiErr.Lookup(context.Background(), "repos/a/missing", nil))
-	require.Zero(t, (&APIError{Status: http.StatusNotFound}).Lookup(context.Background(), "repos/a/b", nil), "errors built without a request cannot look up")
+	lookup := apiErr.LookupClient()
+	require.NotNil(t, lookup)
+	resp, err := lookup.Get(server.URL + "/base/api/v1/repos/a/b")
+	require.NoError(t, err)
+	_ = resp.Body.Close()
+	require.Equal(t, http.StatusOK, resp.StatusCode)
+	require.Nil(t, (&APIError{Status: http.StatusNotFound}).LookupClient(), "errors built without a request cannot look up")
 }