fix(logging): let callers see through the journald formatter wrapper

CI caught what local testing could not: two existing tests in
test_logging_config.py assert that setup_logging() selected a
StructuredFormatter or a ContextualFormatter, by checking the console
handler's formatter directly. Wrapping that formatter to tag each line with
its syslog priority makes those assertions false.

They passed locally and failed on the runner because the wrapper is applied
only when JOURNAL_STREAM is set -- absent in a terminal, present in CI. An
environment-dependent break, which is the kind that gets shipped.

The wrapper now exposes the formatter it delegates to, and those two tests
look through it. They are about which formatter format_type selects, and that
behaviour is unchanged; only the object they have to reach for moved.

Verified both ways this time: 39 tests pass with JOURNAL_STREAM set and with
it unset.

test/test_logging_config.py

@@ -183,15 +183,27 @@ class TestSetupLogging:
         setup_logging()
         assert len(logging.getLogger().handlers) == 1
 
+    @staticmethod
+    def _selected_formatter():
+        """The formatter setup_logging() chose, past any journald wrapper.
+
+        Under systemd the console handler's formatter is wrapped so each line
+        carries its syslog priority. That wrapper is applied only when
+        JOURNAL_STREAM is set, which is true in CI and false in a terminal, so
+        asserting on the handler's formatter directly passes locally and fails
+        on the runner. These tests are about which formatter format_type
+        selects, so they look through the wrapper.
+        """
+        formatter = logging.getLogger().handlers[0].formatter
+        return getattr(formatter, "inner", formatter)
+
     def test_json_format_selects_structured_formatter(self):
         setup_logging(format_type="json")
-        assert isinstance(
-            logging.getLogger().handlers[0].formatter, StructuredFormatter)
+        assert isinstance(self._selected_formatter(), StructuredFormatter)
 
     def test_readable_format_selects_contextual_formatter(self):
         setup_logging(format_type="readable")
-        assert isinstance(
-            logging.getLogger().handlers[0].formatter, ContextualFormatter)
+        assert isinstance(self._selected_formatter(), ContextualFormatter)
 
     def test_log_file_adds_file_handler(self, tmp_path):
         log_file = tmp_path / "test.log"