fix(lerobot): preserve multitask frame labels on export

[harshitethic]

Summary

• preserve each source episode's complete task list instead of collapsing it to the first task during export
• validate every copied frame's task_index against the task list that will actually be published
• preserve task_index in the exported info.json feature schema
• add positive coverage for a two-task episode and a refusal test for an out-of-range frame task pointer

Verification

• git diff --check
• uv run ruff check examples/lerobot/export.py tests/test_lerobot_export.py
• uv run pytest -q tests/test_lerobot_export.py -k 'multitask or task_index_outside'

Fixes #529

[github-actions]

👋 Hi @harshitethic — thanks for the contribution! To keep starter issues available
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You already have #531 open, so this one is being closed automatically.
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