feat: implement comprehensive bridge-agnostic user management system

This commit implements a complete multi-user bridge management system that allows bridges to control multiple users with async goroutine management and convenience methods for channel operations.

Key features:
- Bridge-agnostic BridgeUser interface with validation, identity, state management, channel operations, connection lifecycle, and goroutine lifecycle methods
- BridgeUserManager interface for user lifecycle management with bridge type identification
- XMPPUser implementation for XMPP bridge with XMPP client integration, connection monitoring, and room operations
- MattermostUser implementation for Mattermost bridge with API integration and channel management
- Updated Bridge interface to include GetUserManager() method
- Base UserManager implementation with generic user management logic
- Added Ping() and CheckChannelExists() methods to BridgeUser interface for health checking and room validation
- Updated bridge manager naming from Manager to BridgeManager for clarity

The system enables bridges to manage multiple users (like "Mattermost Bridge" user in XMPP) with proper state management, connection monitoring, and channel operations abstracted across different bridge protocols.

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

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

server/plugin.go

@@ -83,7 +83,7 @@ func (p *Plugin) OnActivate() error {
 	}
 
 	// Initialize bridge manager
-	p.bridgeManager = bridge.NewManager(p.logger, p.API, p.remoteID)
+	p.bridgeManager = bridge.NewBridgeManager(p.logger, p.API, p.remoteID)
 
 	// Initialize and register bridges with current configuration
 	if err := p.initBridges(*cfg); err != nil {