Minor optimization: avoid sprintf in character loop for better performance

Agent-Logs-Url: https://github.com/fsprojects/FSharp.Data.GraphQL/sessions/ce0fba11-9043-452f-b948-e03c8b644f26

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

Author: Copilot

src/FSharp.Data.GraphQL.Shared/AstExtensions.fs

@@ -117,7 +117,9 @@ type Document with
                     | '\n' -> "\\n"
                     | '\r' -> "\\r"
                     | '\t' -> "\\t"
-                    | c when c < '\u0020' -> sprintf "\\u%04X" (int c)
+                    | c when c < '\u0020' -> 
+                        let hex = (int c).ToString("X4", CultureInfo.InvariantCulture)
+                        "\\u" + hex
                     | c -> string c
                 escaped.Append(appendStr) |> ignore
             escaped.Append('"').ToString()