feat: complete XMPP bridge implementation with configuration fixes

- Fix configuration loading by matching JSON field names with plugin manifest keys
- Move configuration to separate package to resolve type conflicts
- Implement bridge startup logic that initializes on OnActivate and updates on OnConfigurationChange
- Add certificate verification skip option for development/testing environments
- Create XMPP client initialization helper function to avoid code duplication
- Add SetOnlinePresence() method to XMPP client for presence management
- Set bridge user online presence automatically upon successful XMPP connection
- Remove unused mock generation and test files as requested
- Update bridge constructor to accept configuration parameter
- Implement proper bridge lifecycle management with Start/Stop methods

The bridge now properly loads configuration from admin console, creates XMPP connections
with appropriate TLS settings, and manages online presence for the bridge user.

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

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

server/model/bridge.go

@@ -0,0 +1,16 @@
+package model
+
+type Bridge interface {
+	// UpdateConfiguration updates the bridge configuration
+	UpdateConfiguration(config any) error
+	// Start starts the bridge
+	Start() error
+	// Stop stops the bridge
+	Stop() error
+	// CreateChannelRoomMapping creates a mapping between a Mattermost channel ID and an bridge room ID.
+	CreateChannelRoomMapping(channelID, roomJID string) error
+	// GetChannelRoomMapping retrieves the bridge room ID for a given Mattermost channel ID.
+	GetChannelRoomMapping(channelID string) (string, error)
+	// IsConnected checks if the bridge is connected to the remote service.
+	IsConnected() bool
+}