Tolerate exceptions in `_flush_loop()`

We saw this exception

```
Traceback (most recent call last):
  File "/app/.venv/lib/python3.14/site-packages/sentry_sdk/_log_batcher.py", line 33, in _to_transport_format
    k: serialize_attribute(v) for (k, v) in item["attributes"].items()
RuntimeError: _core::PyFeatureContext is unsendable, but is being dropped on another thread"
```