fix: queue=True no longer strips exc_info before JsonLinesFormatter sees it

QueueHandler.prepare() formatted the record with its own default formatter and
nulled exc_info/exc_text/stack_info before the listener's real formatter ran,
so queue=True + output="json" produced a JSON line with no exc_info key and
the traceback folded into message instead. _PreservingQueueHandler skips that
premature format-and-strip (safe here - the queue is in-process only, nothing
needs to stay picklable) so the listener's handler formats the untouched
record exactly once, matching the non-queued path.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/log_setup/setup.py

@@ -29,6 +29,22 @@ _listener = None
 _atexit_registered = False
 
 
+class _PreservingQueueHandler(logging.handlers.QueueHandler):
+    """QueueHandler that hands the record to the listener untouched
+
+    stdlib's QueueHandler.prepare() calls self.format(record) with the queue
+    handler's OWN (default) formatter, then nulls exc_info/exc_text/stack_info -
+    losing structured fields (e.g. JsonLinesFormatter's exc_info key) before the
+    listener's real formatter ever sees the record. that behavior exists to keep a
+    record picklable across a multiprocessing.Queue; this lib only ever uses an
+    in-process queue.Queue, so there is nothing to pickle and nothing to strip -
+    the listener's handler formats the untouched record exactly once.
+    """
+
+    def prepare(self, record: logging.LogRecord) -> logging.LogRecord:
+        return record
+
+
 def _exc_text() -> str:
     """render sys.exc_info() as text, for capturing a traceback into a buffered warning"""
     return "".join(traceback.format_exception(*sys.exc_info())).strip()
@@ -284,7 +300,7 @@ def setup_logging(
 
     if queue:
         record_queue: "_queue.Queue" = _make_queue()
-        qh = _tag(logging.handlers.QueueHandler(record_queue))
+        qh = _tag(_PreservingQueueHandler(record_queue))
         root.addHandler(qh)
         _listener = logging.handlers.QueueListener(record_queue, *handlers, respect_handler_level=True)
         _listener.start()