feat: implement XEP-0077 In-Band Registration support

- Add XEPFeatures framework for managing XMPP extension protocols
- Implement complete XEP-0077 In-Band Registration functionality
- Add server capability detection using disco#info queries
- Only initialize XEP features when server supports them
- Add comprehensive XEP-0077 testing to doctor command
- Doctor tests create and delete test users to validate functionality
- Add struct-based XEP management instead of dynamic maps

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

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

server/xmpp/xep_features.go

@@ -0,0 +1,58 @@
+// Package xmpp provides XEP (XMPP Extension Protocol) feature implementations.
+package xmpp
+
+import (
+	"sync"
+
+	"github.com/mattermost/mattermost-plugin-bridge-xmpp/server/logger"
+)
+
+// XEPHandler defines the interface that all XEP implementations must satisfy
+type XEPHandler interface {
+	// Namespace returns the XML namespace for this XEP
+	Namespace() string
+
+	// Name returns a human-readable name for this XEP
+	Name() string
+}
+
+// XEPFeatures manages all XEP implementations for an XMPP client
+type XEPFeatures struct {
+	// XEP-0077: In-Band Registration
+	InBandRegistration *InBandRegistration
+
+	logger logger.Logger
+	mu     sync.RWMutex
+}
+
+// NewXEPFeatures creates a new XEP features manager
+func NewXEPFeatures(logger logger.Logger) *XEPFeatures {
+	return &XEPFeatures{
+		logger: logger,
+	}
+}
+
+// ListFeatures returns a list of available XEP feature names
+func (x *XEPFeatures) ListFeatures() []string {
+	x.mu.RLock()
+	defer x.mu.RUnlock()
+
+	var features []string
+	if x.InBandRegistration != nil {
+		features = append(features, "InBandRegistration")
+	}
+
+	return features
+}
+
+// GetFeatureByNamespace retrieves a XEP feature by its XML namespace
+func (x *XEPFeatures) GetFeatureByNamespace(namespace string) XEPHandler {
+	x.mu.RLock()
+	defer x.mu.RUnlock()
+
+	if x.InBandRegistration != nil && x.InBandRegistration.Namespace() == namespace {
+		return x.InBandRegistration
+	}
+
+	return nil
+}