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>

go.mod

@@ -8,6 +8,8 @@ require (
 	github.com/mattermost/mattermost/server/public v0.1.10
 	github.com/pkg/errors v0.9.1
 	github.com/stretchr/testify v1.10.0
+	mellium.im/sasl v0.3.2
+	mellium.im/xmpp v0.22.0
 )
 
 require (
@@ -59,7 +61,5 @@ require (
 	gopkg.in/yaml.v2 v2.4.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 	mellium.im/reader v0.1.0 // indirect
-	mellium.im/sasl v0.3.2 // indirect
 	mellium.im/xmlstream v0.15.4 // indirect
-	mellium.im/xmpp v0.22.0 // indirect
 )