feat(plugin): private chat messages commands

[GuiLeme]

What does this PR do?

This pull request adds private chat support for BigBlueButton plugins, enabling plugins to programmatically create private chats and interact with them through new server commands and UI commands.

Changes made:

1. Server Commands - Chat:

   • Added sendChatMessage() - Send messages to specific chats by chatId
   • Added createPrivateChat() - Create private chats with specific users

2. UI Commands - Chat Form:

   • Enhanced open() - Now accepts optional chatId parameter to open specific chats

3. Updated Sample Plugin:

   • Updated sample-ui-commands-plugin to demonstrate the new private chat features
   • Shows complete workflow: create private chat, open it, and fill the form

4. Documentation:

   • Updated docs/plugins.md with new private chat commands

Motivation

This PR extends the plugin SDK to support private chat operations, enabling use cases such as:

• Automated private messaging between users
• Custom chat routing logic
• Enhanced collaboration features
• Private notification systems

More

Closely related to: bigbluebutton/bigbluebutton#24200

• Added/updated documentation
• Sample plugin demonstrates complete private chat workflow
• Backward compatible - all existing commands maintain their original behavior
• New commands follow existing SDK patterns and conventions

See demo video ahead:

ui-commands-private-chat.mp4

[github-actions]

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Please resolve those so we can review the pull request.
Thanks.