login: Queue session for garbage collection on leader death (#36364)

This ensures sessions are cleaned up properly in case the user service
manager was manually stopped.

How to reproduce:
- Install Fedora 41
- Log in
- Run `systemctl --user exit`
- Log in again
- Run `loginctl`

Expected results:
- Only 1 `manager` session

Actual results:
- More than 1 `manager` session

src/login/logind-session.c

@@ -91,8 +91,13 @@ static int session_dispatch_leader_pidfd(sd_event_source *es, int fd, uint32_t r
         Session *s = ASSERT_PTR(userdata);
 
         assert(s->leader.fd == fd);
+
+        s->leader_pidfd_event_source = sd_event_source_unref(s->leader_pidfd_event_source);
+
         session_stop(s, /* force= */ false);
 
+        session_add_to_gc_queue(s);
+
         return 1;
 }
 
@@ -959,7 +964,6 @@ int session_stop(Session *s, bool force) {
                 return 0;
 
         s->timer_event_source = sd_event_source_unref(s->timer_event_source);
-        s->leader_pidfd_event_source = sd_event_source_unref(s->leader_pidfd_event_source);
 
         if (s->seat)
                 seat_evict_position(s->seat, s);
@@ -1270,6 +1274,8 @@ static int session_dispatch_fifo(sd_event_source *es, int fd, uint32_t revents,
         session_remove_fifo(s);
         session_stop(s, /* force = */ false);
 
+        session_add_to_gc_queue(s);
+
         return 1;
 }