test: cover the chat access policy and bootstrap guards

ChatAccess admits only chat-group members to context bm/1 and manages
nothing; the chat context oversees nothing and its loader knows only
id 1; /chat and /chat/members need a caller and a database. The
migration count rises to 15 for the chat group seed.

plugin_test.go

@@ -77,8 +77,8 @@ func TestPluginActivates(t *testing.T) {
 	if n := len(p.Models()); n != 11 {
 		t.Errorf("Models = %d, want 11", n)
 	}
-	if n := len(p.Migrations()); n != 14 {
-		t.Errorf("Migrations = %d, want 14", n)
+	if n := len(p.Migrations()); n != 15 {
+		t.Errorf("Migrations = %d, want 15", n)
 	}
 	if !slices.ContainsFunc(p.Commands(), func(c bonfire.Command) bool { return c.Name == "bm:seed" }) {
 		t.Error("bm:seed is not registered")