Use initializer-style extensions in integration error resolver

Agent-Logs-Url: https://github.com/fsprojects/FSharp.Data.GraphQL/sessions/d64ab4ee-eb67-4b7c-a114-d6ad4b640254

Co-authored-by: xperiandri <2365592+xperiandri@users.noreply.github.com>

Author: Copilot

tests/FSharp.Data.GraphQL.IntegrationTests.Server/Schema.fs

@@ -144,9 +144,11 @@ module Schema =
                       args = [],
                       resolve =
                           fun _ _ ->
-                              let extensions = Dictionary<string, obj> 2
-                              extensions["code"] <- box "OPERATION_ERROR_TEST"
-                              extensions["severity"] <- box 7
+                              let extensions =
+                                  Dictionary<string, obj>(
+                                      [ KeyValuePair("code", box "OPERATION_ERROR_TEST")
+                                        KeyValuePair("severity", box 7) ]
+                                  )
                               raise (GQLMessageException("Always fails for tests", extensions))
                   ) ])