feat: implement bidirectional message bridge system with XMPP-Mattermost integration

This commit implements a comprehensive bridge-agnostic message routing system that enables
real-time bidirectional message synchronization between XMPP and Mattermost platforms.

Key features:
- Bridge-agnostic message types and structures for extensibility
- Central message bus system with publisher-subscriber pattern
- Complete Bridge interface implementation for both XMPP and Mattermost
- Message aggregation from multiple sources for scalability
- Loop prevention mechanisms to avoid infinite message cycles
- Buffered channels for high-performance message processing

Architecture highlights:
- Producer-consumer pattern for message routing between bridges
- Thread-safe goroutine lifecycle management with context cancellation
- Message handlers separated into dedicated files for maintainability
- Support for future bridge implementations (Slack, Discord, etc.)
- Markdown content standardization across all bridges

Files added:
- server/model/message.go: Core bridge-agnostic message structures
- server/bridge/messagebus.go: Central message routing system
- server/bridge/mattermost/message_handler.go: Mattermost-specific message processing
- server/bridge/xmpp/message_handler.go: XMPP-specific message processing

Files modified:
- server/bridge/manager.go: Integration with message bus and routing
- server/bridge/mattermost/bridge.go: Complete Bridge interface implementation
- server/bridge/xmpp/bridge.go: Message aggregation and interface completion
- server/model/bridge.go: Extended Bridge interface for bidirectional messaging
- server/xmpp/client.go: Enhanced message listening with mellium.im/xmpp

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

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

server/bridge/mattermost/bridge.go

@@ -14,6 +14,11 @@ import (
 	"github.com/mattermost/mattermost/server/public/plugin"
 )
 
+const (
+	// defaultMessageBufferSize is the buffer size for incoming message channels
+	defaultMessageBufferSize = 1000
+)
+
 // mattermostBridge handles syncing messages between Mattermost instances
 type mattermostBridge struct {
 	logger      logger.Logger
@@ -21,6 +26,11 @@ type mattermostBridge struct {
 	kvstore     kvstore.KVStore
 	userManager pluginModel.BridgeUserManager
 
+	// Message handling
+	messageHandler   *mattermostMessageHandler
+	userResolver     *mattermostUserResolver
+	incomingMessages chan *pluginModel.DirectionalMessage
+
 	// Connection management
 	connected atomic.Bool
 	ctx       context.Context
@@ -38,18 +48,23 @@ type mattermostBridge struct {
 // NewBridge creates a new Mattermost bridge
 func NewBridge(log logger.Logger, api plugin.API, kvstore kvstore.KVStore, cfg *config.Configuration) pluginModel.Bridge {
 	ctx, cancel := context.WithCancel(context.Background())
-	bridge := &mattermostBridge{
-		logger:          log,
-		api:             api,
-		kvstore:         kvstore,
-		ctx:             ctx,
-		cancel:          cancel,
-		channelMappings: make(map[string]string),
-		config:          cfg,
-		userManager:     bridge.NewUserManager("mattermost", log),
+	b := &mattermostBridge{
+		logger:           log,
+		api:              api,
+		kvstore:          kvstore,
+		ctx:              ctx,
+		cancel:           cancel,
+		channelMappings:  make(map[string]string),
+		config:           cfg,
+		userManager:      bridge.NewUserManager("mattermost", log),
+		incomingMessages: make(chan *pluginModel.DirectionalMessage, defaultMessageBufferSize),
 	}
 
-	return bridge
+	// Initialize handlers after bridge is created
+	b.messageHandler = newMessageHandler(b)
+	b.userResolver = newUserResolver(b)
+
+	return b
 }
 
 // getConfiguration safely retrieves the current configuration
@@ -343,3 +358,23 @@ func (b *mattermostBridge) GetRoomMapping(roomID string) (string, error) {
 func (b *mattermostBridge) GetUserManager() pluginModel.BridgeUserManager {
 	return b.userManager
 }
+
+// GetMessageChannel returns the channel for incoming messages from Mattermost
+func (b *mattermostBridge) GetMessageChannel() <-chan *pluginModel.DirectionalMessage {
+	return b.incomingMessages
+}
+
+// SendMessage sends a message to a Mattermost channel
+func (b *mattermostBridge) SendMessage(msg *pluginModel.BridgeMessage) error {
+	return b.messageHandler.postMessageToMattermost(msg)
+}
+
+// GetMessageHandler returns the message handler for this bridge
+func (b *mattermostBridge) GetMessageHandler() pluginModel.MessageHandler {
+	return b.messageHandler
+}
+
+// GetUserResolver returns the user resolver for this bridge
+func (b *mattermostBridge) GetUserResolver() pluginModel.UserResolver {
+	return b.userResolver
+}