ansible_mitogen: Return stderr_lines from _low_level_execute_command()

Vanilla Ansible has returned stderr since v1.9 or earlier, stderr_lines was
added in v2.6.0 (https://github.com/ansible/ansible/pull/40079).

ansible_mitogen/mixins.py

@@ -491,10 +491,12 @@ class ActionModuleMixin(ansible.plugins.action.ActionBase):
                     continue
 
         stdout_text = to_text(stdout, errors=encoding_errors)
+        stderr_text = to_text(stderr, errors=encoding_errors)
 
         return {
             'rc': rc,
             'stdout': stdout_text,
             'stdout_lines': stdout_text.splitlines(),
-            'stderr': stderr,
+            'stderr': stderr_text,
+            'stderr_lines': stderr_text.splitlines(),
         }

docs/changelog.rst

@@ -32,6 +32,8 @@ In progress (unreleased)
   attributes
 * :gh:issue:`1213` :mod:`ansible_mitogen`: Decouple possible_pythons order &
   error handling
+* :gh:issue:`1213` :mod:`ansible_mitogen`: Return ``stderr_lines`` from
+  ``_low_level_execute_command()``
 
 
 v0.3.21 (2025-01-20)