Closes #46846

When `disable_ai: true` is set in user settings, Zed was still
connecting to configured MCP (context) servers and sending
initialization requests. This change adds checks for `DisableAiSettings`
in `ContextServerStore` to:

- Skip server connections when AI is disabled
- Disconnect from running servers when AI becomes disabled
- Connect to servers when AI is re-enabled
- Prevent registry changes from triggering connections while AI is
disabled

The fix tracks `ai_disabled` state to detect transitions and properly
manage server connections when AI is toggled.

Release Notes:

- Fixed Zed connecting to MCP servers when AI is disabled.

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Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>