feat(edit-dataset): add concatenate_videos opt-out to merge (#3663)

* feat(edit-dataset): add `concatenate_videos` opt-out to merge

When merging datasets, source mp4s are concatenated into shards capped at
`video_files_size_in_mb` (default 200 MB). This is great for dataloader
throughput but destroys per-episode (or per-source) video boundaries,
which is undesirable when you want to inspect, ship, or reuse the
individual mp4s.

Add a `concatenate_videos: bool = True` knob plumbed through
`MergeConfig` → `merge_datasets` → `aggregate_datasets` → `aggregate_videos`.
When False, each source mp4 is copied 1:1 to its own destination mp4 with
no re-muxing, so the merge preserves source video boundaries.

Usage:

    lerobot-edit-dataset \
        --new_repo_id user/merged \
        --operation.type=merge \
        --operation.repo_ids "['user/a', 'user/b']" \
        --operation.concatenate_videos=false

Defaults are unchanged; the dataloader path is unaffected because the
`episodes.parquet` `from_timestamp`/`to_timestamp` index keeps working
regardless of whether each mp4 holds one or many episodes.

* feat(edit-dataset): extend concatenate opt-out to data files

Following review, add a concatenate_data flag mirroring concatenate_videos,
threaded through MergeConfig, merge_datasets, aggregate_datasets, aggregate_data
and append_or_create_parquet_file. Metadata index files still always concatenate.

Also trim the verbose docstrings and comments since the names are
self-explanatory, and extend the existing merge test to cover data files.

tests/scripts/test_edit_dataset_parsing.py

@@ -66,6 +66,20 @@ class TestOperationTypeParsing:
         with pytest.raises(ValueError, match="--new_repo_id is required for merge"):
             _validate_config(cfg)
 
+    @pytest.mark.parametrize("flag", ["concatenate_videos", "concatenate_data"])
+    def test_merge_concatenate_flag_defaults_true(self, flag):
+        cfg = parse_cfg(["--new_repo_id", "test/merged", "--operation.type", "merge"])
+        assert isinstance(cfg.operation, MergeConfig)
+        assert getattr(cfg.operation, flag) is True
+
+    @pytest.mark.parametrize("flag", ["concatenate_videos", "concatenate_data"])
+    def test_merge_concatenate_flag_can_be_disabled(self, flag):
+        cfg = parse_cfg(
+            ["--new_repo_id", "test/merged", "--operation.type", "merge", f"--operation.{flag}", "false"]
+        )
+        assert isinstance(cfg.operation, MergeConfig)
+        assert getattr(cfg.operation, flag) is False
+
     def test_non_merge_requires_repo_id(self):
         cfg = parse_cfg(["--operation.type", "delete_episodes"])
         with pytest.raises(ValueError, match="--repo_id is required for delete_episodes"):