Fix Selectable Tools

[MikeAlhayek]

This pull request improves the handling and visibility of AI tools and agents, ensuring that only user-selectable tools and agents appear in UI pickers and public APIs, while hidden and system-managed components remain accessible only where appropriate. It introduces new APIs and updates documentation to clarify these behaviors, and tightens enforcement across the codebase and server handlers.

Key changes:

Core logic and API enhancements

• Added IsSelectable() to AIToolDefinitionEntry and GetSelectableTools() to AIToolDefinitionOptions, enabling clear separation of selectable tools from hidden/system tools. [1] [2]
• Updated ChatInteractionHubBase.ApplyCoreSettingsAsync to persist only selectable tools and user-selectable agents, filtering out hidden/system ones during chat interaction setup. [1] [2]
• Improved IsUserSelectableAgent() extension to exclude both always-available and system agents from UI selection.

Server handler restrictions

• Modified MCP server handlers to exclude hidden tools from tool listing and invocation, ensuring only public tools are exposed to clients. [1] [2]

Documentation and release notes

• Updated documentation throughout to clarify the distinction between selectable, hidden, and system tools/agents, including new patterns for system agents that are not user-selectable but participate in orchestration and remote APIs. [1] [2] [3] [4] [5] [6] [7] [8] [9]
• Updated release notes to reflect tightened selection flows, keeping hidden/system tools and agents out of manual pickers and public APIs. [1] [2]

These changes collectively ensure clearer boundaries between user-facing and internal AI components, improving security, maintainability, and user experience.