Fix test collection and harness teardown on CI runners

The characterization harness is a session leader whose controlling terminal
is the PTY it opens; Linux delivers SIGHUP when the master closes, killing it
before the report is written. Ignore the hangup — it is teardown noise.

test_terminal_process imported termios at module level, breaking collection
on Windows; move it behind the existing platform guard.

Author: gorandodig

tests/test_pty_characterization.py

@@ -78,13 +78,19 @@
 import fcntl
 import json
 import os
+import signal
 import subprocess
 import sys
 import termios
 
 PROBE_TIMEOUT_SECONDS = 20
 CLEANUP_TIMEOUT_SECONDS = 10
 
+# Closing the PTY master hangs up the terminal; Linux delivers SIGHUP to this
+# process (session leader with the slave as controlling terminal) before the
+# report is written. The hangup is teardown noise, not part of the topology.
+signal.signal(signal.SIGHUP, signal.SIG_IGN)
+
 
 def reap(process):
     process.kill()

tests/test_terminal_process.py

@@ -11,7 +11,6 @@
 import signal
 import subprocess
 import sys
-import termios
 import threading
 import time
 from contextlib import contextmanager
@@ -24,6 +23,8 @@
 pytestmark = posix_only
 
 if sys.platform != "win32":
+    import termios
+
     from render_machine import pty_exec
 
 REPO_ROOT = Path(__file__).resolve().parent.parent