Fix JSON test comparison to handle Unicode escape differences

System.Text.Json serializes supplementary-plane characters (e.g. U+29E3D)
as \uD867\uDE3D escape sequences, while SQL Server returns them as literal
UTF-8. The DeepEqualsCore fallback compared GetRawText() which preserves
escaping differences. Add explicit JsonValueKind.String case using
GetString() which decodes both representations to the same .NET string.

Author: paulmedynski

src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/JsonTest/JsonTestHelper.cs

@@ -73,6 +73,9 @@ private static bool DeepEqualsCore(JsonElement a, JsonElement b, int depth)
                     }
                     return !arrB.MoveNext();
 
+                case JsonValueKind.String:
+                    return a.GetString() == b.GetString();
+
                 default:
                     return a.GetRawText() == b.GetRawText();
             }