feat: add legacy agents packages support

[andreiancuta-uipath]

• Processes legacy messages and resources in a model validator
• Default outputSchema to an empty schema -- there are older packages without outputSchema
• Fixed: escalationType was not defaulting to 0 when the field was absent
• I observed a recipientType that was all lowercase -> I made recipientType normalization case-insensitive
• recipientType enum "GroupName" is different from the C# enum "StaticGroupName". Deserialization fails only in published agents, because playground sends integer enums.
  • Fixed by mapping "StaticGroupName" to "GroupName" in normalize_recipient_type. The alternative is to update the Python enum, but that is a breaking change and requires a major version bump.