Added test for increase in LearnerHandler Queue despite shutdown invocation

[mudit-saxena]

 * Tests that the packet queue can grow during a slow/blocked shutdown process.
 * 
 * This test verifies a specific race condition scenario:
 * 1. A background thread continuously adds packets to the learner's queue
 * 2. Shutdown is initiated, which clears the queue and adds proposalOfDeath
 * 3. Shutdown then blocks on socket.close() (mocked to take 5 seconds)
 * 4. During the socket close wait, the background thread continues adding packets
 * 5. The queue grows even though shutdown has been initiated
 * 
 * This simulates real-world scenarios where:
 * - Network I/O is slow during shutdown
 * - The leader continues sending packets while learner is shutting down
 * - The queue can unexpectedly grow during the shutdown process