feat: refactor bridge user detection and fix linting issues

- Extract bridge user detection logic into separate isBridgeUserMessage() function
- Fix gocritic ifElseChain issues by converting if-else to switch statements
- Fix gofmt formatting issues in client.go
- Fix revive naming issues by renaming XMPPUser to User and XMPPUserManager to UserManager
- Improve code organization and maintainability

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

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

server/hooks_user.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/plugin"
+)
+
+// UserHasBeenDeleted is called when a user has been deleted from Mattermost
+// This allows us to clean up ghost users from the XMPP server
+func (p *Plugin) UserHasBeenDeleted(c *plugin.Context, user *model.User) {
+	if user == nil {
+		p.logger.LogWarn("UserHasBeenDeleted called with nil user")
+		return
+	}
+
+	p.logger.LogDebug("User deleted from Mattermost, cleaning up bridge users", "user_id", user.Id, "username", user.Username)
+
+	// Clean up ghost users from external bridges (skip Mattermost bridge)
+	for _, bridgeName := range p.bridgeManager.ListBridges() {
+		// Skip the Mattermost bridge since it represents the Mattermost side
+		if bridgeName == "mattermost" {
+			p.logger.LogDebug("Skipping Mattermost bridge for user cleanup", "user_id", user.Id)
+			continue
+		}
+
+		bridge, err := p.bridgeManager.GetBridge(bridgeName)
+		if err != nil {
+			p.logger.LogWarn("Failed to get bridge for user cleanup", "bridge", bridgeName, "error", err)
+			continue
+		}
+
+		userManager := bridge.GetUserManager()
+		if userManager == nil {
+			p.logger.LogDebug("Bridge has no user manager, skipping cleanup", "bridge", bridgeName)
+			continue
+		}
+
+		// Check if this user exists in the bridge
+		if !userManager.HasUser(user.Id) {
+			p.logger.LogDebug("User not found in bridge, skipping cleanup", "bridge", bridgeName, "user_id", user.Id)
+			continue
+		}
+
+		// Delete the user from the bridge (this will handle ghost user cleanup if enabled)
+		if err := userManager.DeleteUser(user.Id); err != nil {
+			p.logger.LogWarn("Failed to delete user from bridge", "bridge", bridgeName, "user_id", user.Id, "error", err)
+		} else {
+			p.logger.LogInfo("Successfully deleted user from bridge", "bridge", bridgeName, "user_id", user.Id, "username", user.Username)
+		}
+	}
+}