MCP: Open User Configuration should offer all user locations #342507

Testing #342409

Run the command MCP: Open User Configuration, it opens the legacy location (VS Code user data)

It should instead open a quick pick with all valid user locations for that harness.

• It should make the legacy location as 'legacy'
• It should have the recommended option first
• It should also show non existing files, marked as 'not existing' and offer to create them when selected
• it should have an option 'Open MCP customizations' entry or header action that opens the MCP page

It would be nice if the command is also mentioned or available in the chat customization dialog.