feat: implement comprehensive room validation and admin-only command access

- Add RoomExists and GetRoomMapping methods to Bridge interface
- Implement XMPP room existence checking using disco#info queries (XEP-0030)
- Add room validation in BridgeManager to prevent duplicate mappings and invalid rooms
- Enhance XMPP client with CheckRoomExists method and comprehensive logging
- Implement admin-only access control for all bridge commands
- Add user-friendly error messages with actionable troubleshooting steps
- Update doctor command with room existence testing and pre-join validation
- Add SimpleLogger implementation for standalone command usage

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

server/model/bridge.go

@@ -133,6 +133,12 @@ type Bridge interface {
 	// DeleteChannelMapping removes a mapping between a Mattermost channel ID and a bridge room ID.
 	DeleteChannelMapping(channelID string) error
 
+	// RoomExists checks if a room/channel exists on the remote service.
+	RoomExists(roomID string) (bool, error)
+
+	// GetRoomMapping retrieves the Mattermost channel ID for a given room ID (reverse lookup).
+	GetRoomMapping(roomID string) (string, error)
+
 	// IsConnected checks if the bridge is connected to the remote service.
 	IsConnected() bool
 }