fix(dataset): adding metadata loading when reading from a dataset after writing (#3305)

* fix(one shot load): adding metadata loading when reading from a dataset after writing

* refactor(one shot load): move metadata reload to ensure_readable() on LeRobotDatasetMetadata

Move the metadata reload from DatasetReader.load_and_activate() to a new
public ensure_readable() method on LeRobotDatasetMetadata, called from
LeRobotDataset._ensure_reader(). This places lifecycle management in the
right layer: metadata owns its readiness check, the dataset orchestrates
the write-to-read transition, and the reader stays clean.

Also adds a regression test using delta_timestamps to exercise the
meta.episodes access path in the create -> write -> finalize -> read flow.

Co-authored-by: Steven Palma <imstevenpmwork@users.noreply.github.com>

---------

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Steven Palma <imstevenpmwork@users.noreply.github.com>

src/lerobot/datasets/lerobot_dataset.py

@@ -278,6 +278,7 @@ class LeRobotDataset(torch.utils.data.Dataset):
     def _ensure_reader(self) -> DatasetReader:
         """Lazily create the reader on first access."""
         if self.reader is None:
+            self.meta.ensure_readable()
             self.reader = DatasetReader(
                 meta=self.meta,
                 root=self.root,