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/model/bridge.go

@@ -1,6 +1,11 @@
 package model
 
-import "fmt"
+import (
+	"context"
+	"fmt"
+
+	"github.com/mattermost/mattermost-plugin-bridge-xmpp/server/config"
+)
 
 type BridgeID string
 
@@ -144,27 +149,59 @@ type Bridge interface {
 
 	// Ping actively tests the bridge connection health by sending a lightweight request.
 	Ping() error
+
+	// GetUserManager returns the user manager for this bridge.
+	GetUserManager() BridgeUserManager
 }
 
+// BridgeUser represents a user connected to any bridge service
+type BridgeUser interface {
+	// Validation
+	Validate() error
+
+	// Identity (bridge-agnostic)
+	GetID() string
+	GetDisplayName() string
+
+	// State management
+	GetState() UserState
+	SetState(state UserState) error
+
+	// Channel operations (abstracted from rooms/channels/groups)
+	JoinChannel(channelID string) error
+	LeaveChannel(channelID string) error
+	SendMessageToChannel(channelID, message string) error
+
+	// Connection lifecycle
+	Connect() error
+	Disconnect() error
+	IsConnected() bool
+	Ping() error
+
+	// Channel existence check
+	CheckChannelExists(channelID string) (bool, error)
+
+	// Goroutine lifecycle
+	Start(ctx context.Context) error
+	Stop() error
+}
+
+// BridgeUserManager manages users for a specific bridge
 type BridgeUserManager interface {
-	// CreateUser creates a new user in the bridge system.
-	CreateUser(userID string, userData any) error
-
-	// GetUser retrieves user data for a given user ID.
-	GetUser(userID string) (any, error)
-
-	// UpdateUser updates user data for a given user ID.
-	UpdateUser(userID string, userData any) error
-
-	// DeleteUser removes a user from the bridge system.
+	// User lifecycle
+	CreateUser(user BridgeUser) error
+	GetUser(userID string) (BridgeUser, error)
 	DeleteUser(userID string) error
-
-	// ListUsers returns a list of all users in the bridge system.
-	ListUsers() ([]string, error)
-
-	// HasUser checks if a user exists in the bridge system.
+	ListUsers() []BridgeUser
 	HasUser(userID string) bool
 
-	// OnUserStateChange is called when a user's state changes (e.g., online, away, offline).
-	OnUserStateChange(userID string, state UserState) error
+	// Manager lifecycle
+	Start(ctx context.Context) error
+	Stop() error
+
+	// Configuration updates
+	UpdateConfiguration(config *config.Configuration) error
+
+	// Bridge type identification
+	GetBridgeType() string
 }