fix: deadlock using IRB integration on Rails applications

   When using `debugger` in a rails console using IRB integration, the second `continue` causes a fatal deadlock:
   "No live threads left. Deadlock?"

   The root cause is that `leave_subsession` pops `@subsession_stack` and sets `@tc = nil`
   before `thread_stopper.disable` takes full effect. In that window, the
   thread_stopper TracePoint fires on the main thread (back in IRB's eval loop),
   and since `@tc` is nil, the guard `tc == @tc` no longer protects it. The main
   thread is paused via `on_pause`, but the session server is already blocked on
   `@q_evt.pop` -- neither can wake the other.

   The fix for this is addiing `next unless in_subsession?` as the first guard in thread_stopper.
   Once `leave_subsession` has popped the stack, in_subsession? returns false
   and the TracePoint becomes a no-op even if it hasn't been disabled yet.

Author: jvlara

lib/debug/session.rb

@@ -1678,6 +1678,15 @@ def get_thread_client th = Thread.current
 
     private def thread_stopper
       TracePoint.new(:line) do
+        # When leave_subsession pops the subsession stack and sets @tc = nil,
+        # there is a window before the TracePoint is disabled where this block
+        # can still fire on other threads. Without this guard, the main thread
+        # (back in IRB's eval loop) would be paused by on_pause, but the
+        # session server is already waiting on @q_evt.pop -- causing a mutual
+        # deadlock ("No live threads left. Deadlock?").
+        # See: https://github.com/ruby/debug/issues/1158
+        next unless in_subsession?
+
         # run on each thread
         tc = ThreadClient.current
         next if tc.management?