fix(lerobot): preserve full task lists when exporting multi-task episodes

[harshitethic]

Summary

• retain each source episode's full tasks list while reconstructing the materialized LeRobot corpus
• validate frame task_index values against that full list instead of the collapsed provenance task
• publish the same source task list in the exported episode metadata
• add a regression test with frame indexes 0 and 1 resolving to two source tasks

The existing invalid-index regression remains meaningful: a frame still fails before output when its index is outside the source episode's published task list.

Fixes #632.

[github-actions]

👋 Hi @harshitethic — thanks for the contribution! To keep starter issues available
for other contributors and give every pull request a real review, we accept
1 open pull request per contributor at a time.

You already have #651 open, so this one is being closed automatically.
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[greptile-apps]

Confidence Score: 5/5

[Medium risk] Fixes task list handling in episode export logic.

The PR appears safe to merge; no actionable issue was established.

Summary

LeRobot export now preserves each selected episode’s full source task list and checks frame indexes against it.

• Exported episode metadata keeps every source task.
• Frames can keep indexes that point to any task in that list.

_{Reviews (1) · Last reviewed commit: "test(lerobot): cover multi-task source i..."}