tests: fix command-queue race in mock-ssh-server exec handling (#72)

Handler.run() creates the per-channel command queue with a
check-then-assign while paramiko's transport thread creates it with
setdefault() and puts the command into it from
check_channel_exec_request(). When the exec request lands inside the
window, run() replaces the queue that already holds the command,
handle_client() blocks on Queue.get() forever and the client never
receives an exit status.

This is the intermittent CI hang: every _execute()-based operation
(cp_file, checksum, the move fallback) rolls these dice, and
test_concurrency_for_raw_commands rolls them 16 at a time. Confirmed
by the faulthandler dump from the first run with #71 merged
(handle_client threads parked on Queue.get with the client waiting),
and reproduced deterministically by widening the window with a 5ms
sleep: upstream logic deadlocks on the first round, the same logic
with an atomic setdefault() survives, with or without the delay.
mock-ssh-server is unmaintained, so the method is patched in conftest
instead of upstream.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: shcheklein

tests/conftest.py

@@ -0,0 +1,28 @@
+import threading
+from queue import Queue
+
+import mockssh.server
+
+
+def _handler_run(self):
+    # Identical to mockssh.server.Handler.run except that the command
+    # queue is created atomically. Upstream checks `chanid not in
+    # command_queues` and then assigns a fresh Queue, while paramiko's
+    # transport thread may concurrently create the queue and put the
+    # command into it via check_channel_exec_request(); the assignment
+    # then replaces that queue, the command is lost, and handle_client
+    # blocks on Queue.get() forever -- the client never receives an
+    # exit status. mock-ssh-server is unmaintained, so it is patched
+    # here instead of upstream.
+    self.transport.start_server(server=self)
+    while True:
+        channel = self.transport.accept()
+        if channel is None:
+            break
+        self.command_queues.setdefault(channel.chanid, Queue())
+        thread = threading.Thread(target=self.handle_client, args=(channel,))
+        thread.daemon = True
+        thread.start()
+
+
+mockssh.server.Handler.run = _handler_run