AI enhanced search interface

[jespino]

Description

This PR adds an AI-powered search interface to the Mattermost AI plugin, allowing users to perform natural language searches across their accessible channels and messages.

Key changes include:

• Added a new search endpoint in the server API that creates AI-powered search conversations
• Integrated with Mattermost's search UI by registering custom search components:
  • Custom search button with Copilot branding
  • Search suggestions showing recent searches and AI-powered suggestions
  • Search hints explaining the Copilot search capabilities
• Enhanced the LLMBot post component to properly display search results with question/answer formatting
• Added license checks to ensure the search feature is only available with appropriate licensing
• Added i18n support for search-related strings

The search interface allows users to:

• Ask natural language questions about content in their channels
• See suggested AI-powered search queries
• View results in a conversational format with clear question/answer separation
• Access their recent search history

This feature requires an enterprise license to use.

Testing Notes

1. Verify search interface appears with Copilot branding
2. Test natural language searches work and return relevant results
3. Verify suggested searches work when clicked
4. Confirm search results show proper question/answer formatting
5. Verify feature is properly gated behind enterprise license
6. Test with non-English locales to verify i18n works correctly