Add plugin message fallback

Route unmatched messages through plugin fallback handlers

Add observer coverage and bump version to rc.15

plugins.go

@@ -171,7 +171,8 @@ type Plugin[T AppData] struct {
 	skipAutoCmd bool                         // If true, all commands in this plugin are excluded from auto-help
 	logger      *slog.Logger
 
-	handlers map[tgapi.UpdateType]CommandExecutor[T]
+	messageFallback CommandExecutor[T]
+	handlers        map[tgapi.UpdateType]CommandExecutor[T]
 
 	onClose func() error
 }
@@ -243,12 +244,6 @@ func (p *Plugin[T]) AddScene(scene *Scene[T]) *Plugin[T] {
 	return p
 }
 
-// UsePolicy registers a Policy as plugin middleware for all plugin handlers.
-func (p *Plugin[T]) UsePolicy(name string, policy Policy[T]) *Plugin[T] {
-	mw := RequirePolicy(name, policy)
-	return p.AddMiddleware(mw)
-}
-
 // NewScene creates, registers, and returns a new scene owned by the plugin.
 func (p *Plugin[T]) NewScene(name string) *Scene[T] {
 	scene := NewScene[T](name)
@@ -257,6 +252,12 @@ func (p *Plugin[T]) NewScene(name string) *Scene[T] {
 	return scene
 }
 
+// UsePolicy registers a Policy as plugin middleware for all plugin handlers.
+func (p *Plugin[T]) UsePolicy(name string, policy Policy[T]) *Plugin[T] {
+	mw := RequirePolicy(name, policy)
+	return p.AddMiddleware(mw)
+}
+
 // AddUpdateHandler registers a handler for a non-command update type.
 // Message, channel post, and callback query updates stay on the command/payload flow.
 func (p *Plugin[T]) AddUpdateHandler(t tgapi.UpdateType, handler CommandExecutor[T]) *Plugin[T] {
@@ -316,6 +317,13 @@ func (p *Plugin[T]) SetOnClose(f func() error) *Plugin[T] {
 	return p
 }
 
+// SetMessageFallback registers a fallback handler for messages that do not
+// match a command.
+func (p *Plugin[T]) SetMessageFallback(handler CommandExecutor[T]) *Plugin[T] {
+	p.messageFallback = handler
+	return p
+}
+
 // Close releases plugin-owned resources such as its logger and optional
 // OnClose callback.
 func (p *Plugin[T]) Close() error {