default _ansible_tracebacks_for to [] when not specified (#85427)

* eases module unit tests that aren't using our fixture

lib/ansible/module_utils/_internal/_traceback.py

@@ -80,7 +80,7 @@ def _is_module_traceback_enabled(event: TracebackEvent) -> bool:
             from ..basic import _PARSED_MODULE_ARGS
 
             _module_tracebacks_enabled_events = frozenset(
-                TracebackEvent[value.upper()] for value in _PARSED_MODULE_ARGS.get('_ansible_tracebacks_for')
+                TracebackEvent[value.upper()] for value in _PARSED_MODULE_ARGS.get('_ansible_tracebacks_for', [])
             )  # type: ignore[union-attr]
         except BaseException:
             return True  # if things failed early enough that we can't figure this out, assume we want a traceback for troubleshooting

test/units/module_utils/_internal/test_traceback.py

@@ -0,0 +1,31 @@
+from __future__ import annotations
+
+import pytest
+import pytest_mock
+
+from ansible.module_utils._internal import _traceback
+
+
+@pytest.mark.parametrize("patched_parsed_args, event, expected", (
+    (dict(_ansible_tracebacks_for=["error", "warning"]), _traceback.TracebackEvent.ERROR, True),  # included value
+    (dict(_ansible_tracebacks_for=["error", "warning"]), _traceback.TracebackEvent.WARNING, True),  # included value
+    (dict(_ansible_tracebacks_for=["error", "warning"]), _traceback.TracebackEvent.DEPRECATED, False),  # excluded value
+    ({}, _traceback.TracebackEvent.ERROR, False),  # unspecified defaults to no tracebacks
+    (dict(_ansible_tracebacks_for="bogus,values"), _traceback.TracebackEvent.ERROR, True),  # parse failure defaults to always enabled
+    (None, _traceback.TracebackEvent.ERROR, True),  # fetch failure defaults to always enabled
+), ids=str)
+def test_default_module_traceback_config(
+        patched_parsed_args: dict | None,
+        event: _traceback.TracebackEvent,
+        expected: bool,
+        mocker: pytest_mock.MockerFixture
+) -> None:
+    """Validate MU traceback config behavior (including unconfigured/broken config fallbacks)."""
+    from ansible.module_utils import basic
+
+    mocker.patch.object(basic, '_PARSED_MODULE_ARGS', patched_parsed_args)
+
+    # this should just be an importlib.reload() on _traceback, but that redeclares the enum type and breaks the world
+    mocker.patch.object(_traceback, '_module_tracebacks_enabled_events', None)
+
+    assert _traceback._is_module_traceback_enabled(event=event) is expected