feat: implement XMPP bridge configuration and logger setup

- Add comprehensive XMPP settings schema to plugin.json
- Implement configuration struct with validation and helper methods
- Add configurable username prefix for XMPP users
- Set up logger in Plugin struct following Matrix bridge pattern
- Update KV store constants to use XMPP terminology
- Replace Matrix references with XMPP equivalents in test helpers

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

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

server/plugin.go

@@ -27,6 +27,9 @@ type Plugin struct {
 	// commandClient is the client used to register and execute slash commands.
 	commandClient command.Command
 
+	// logger is the main plugin logger
+	logger Logger
+
 	backgroundJob *cluster.Job
 
 	// configurationLock synchronizes access to the configuration.
@@ -41,6 +44,9 @@ type Plugin struct {
 func (p *Plugin) OnActivate() error {
 	p.client = pluginapi.NewClient(p.API, p.Driver)
 
+	// Initialize the logger using Mattermost Plugin API
+	p.logger = NewPluginAPILogger(p.API)
+
 	p.kvstore = kvstore.NewKVStore(p.client)
 
 	p.commandClient = command.NewCommandHandler(p.client)