server/group: refactor with shared abstractions and fix concurrency issues (#5222)

* server/group: refactor group package with shared abstractions and fix concurrency issues

Extract common patterns into reusable components:
- groupRegistry[G]: generic concurrent map for group lifecycle management
- baseGroup: shared plumbing for listener-based groups (TCP, HTTPS, TCPMux)
- Listener: unified virtual listener replacing 3 identical implementations

Fix concurrency issues:
- Stale-pointer race: isCurrent check + errGroupStale + controller retry loops
- Worker generation safety: pass realLn and acceptCh as params instead of reading mutable fields
- Connection leak: close conn on worker panic recovery path
- ABBA deadlock in HTTP UnRegister: consistent lock ordering (group.mu -> registry.mu)
- Round-robin overflow in HTTPGroup: use unsigned modulo

Add unit tests (17 tests) for registry, listener, and baseGroup.
Add TCPMux group load balancing e2e test.

* server/group: replace tautological assertion with require.NotPanics

* server/group: remove blank line between doc comment and type declaration

server/group/registry.go

@@ -0,0 +1,59 @@
+package group
+
+import (
+	"sync"
+)
+
+// groupRegistry is a concurrent map of named groups with
+// automatic creation on first access.
+type groupRegistry[G any] struct {
+	groups map[string]G
+	mu     sync.Mutex
+}
+
+func newGroupRegistry[G any]() groupRegistry[G] {
+	return groupRegistry[G]{
+		groups: make(map[string]G),
+	}
+}
+
+func (r *groupRegistry[G]) getOrCreate(key string, newFn func() G) G {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	g, ok := r.groups[key]
+	if !ok {
+		g = newFn()
+		r.groups[key] = g
+	}
+	return g
+}
+
+func (r *groupRegistry[G]) get(key string) (G, bool) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	g, ok := r.groups[key]
+	return g, ok
+}
+
+// isCurrent returns true if key exists in the registry and matchFn
+// returns true for the stored value.
+func (r *groupRegistry[G]) isCurrent(key string, matchFn func(G) bool) bool {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	g, ok := r.groups[key]
+	return ok && matchFn(g)
+}
+
+// removeIf atomically looks up the group for key, calls fn on it,
+// and removes the entry if fn returns true.
+func (r *groupRegistry[G]) removeIf(key string, fn func(G) bool) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	g, ok := r.groups[key]
+	if !ok {
+		return
+	}
+	if fn(g) {
+		delete(r.groups, key)
+	}
+}