Fix GroupPersona serialization and group chat functionality issues

[Copilot]

This PR addresses several critical issues with the GroupPersona implementation that were causing problems with group chat functionality while maintaining full backward compatibility with 1-on-1 conversations.

Issues Fixed

1. Nested JSON Serialization Problem

The original implementation stored BotPersonas as a List<BasePersona>, which created "nested dolls" when serialized to JSON:

{
  "BotPersonas": [
    {
      "Name": "Alice",
      "Bio": "...",
      "UniqueName": "alice_bot",
      // ... entire persona object nested here
    }
  ]
}

This made individual personas difficult to edit independently and violated the principle of keeping personas as separate entities.

Fix: Added [JsonIgnore] to BotPersonas and introduced BotPersonaNames as a List<string> for serialization:

{
  "BotPersonaNames": ["alice_bot", "bob_bot"],
  "CurrentBotId": "bob_bot"
  // No nested persona objects
}

2. Dynamic Persona Loading

The group now dynamically populates BotPersonas during BeginChat() using LLMSystem.LoadedPersonas, ensuring personas remain independent while being properly linked at runtime.

3. Brain Functionality Conflicts

Brain functionality was interfering with group conversations since it's designed for single personas. Added automatic Brain disabling for group chats:

private bool IsBrainDisabled => LLMSystem.IsGroupConversation;

public void OnUserPost(string userinput)
{
    if (IsBrainDisabled)
        return;
    // ... rest of brain logic
}

4. Enhanced World Info Handling

Improved UpdateRagAndInserts() to consider both group-level and active persona's individual MyWorlds:

// Add world entries from the group/bot itself
if (Bot.MyWorlds.Count > 0) { /* ... */ }

// If in group conversation, also add world entries from current active persona
if (IsGroupConversation)
{
    var currentBot = GetCurrentGroupBot();
    if (currentBot?.MyWorlds.Count > 0) { /* ... */ }
}

Testing

Added comprehensive tests covering:

• Serialization/deserialization cycles
• Dynamic persona loading from LoadedPersonas
• Brain disable functionality
• Group macro resolution
• World info handling

All tests pass and confirm that:

• ✅ No nested personas in JSON
• ✅ Individual personas remain editable
• ✅ Group functionality works correctly
• ✅ 1-on-1 chat compatibility maintained
• ✅ Brain conflicts resolved
• ✅ Enhanced MyWorlds support

Breaking Changes

None. All changes are backward compatible with existing 1-on-1 chat functionality.

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