fix: use thread timeout method for pytest-xdist compatibility

signal-based timeouts corrupt xdist worker processes, causing mass
test failures on CI. thread method is the documented alternative
for parallel test execution.

pytest.ini

@@ -2,6 +2,7 @@
 testpaths = make_post_sell
 python_files = test*.py
 timeout = 60
+timeout_method = thread
 # Uncomment filterwarnings to suppress external library warnings
 # filterwarnings =
 #     # Ignore datetime.utcnow() deprecation from external libraries