[python] Preserve LeRobot recording metadata in direct writer

[YannByron]

Summary

Follow up on #9665 by preserving native LeRobot recording metadata when frames are captured directly into Paimon. The writer now publishes a managed table group containing frames, episodes, tasks, optional subtasks, dataset information, and statistics without creating an intermediate LeRobot dataset directory.

Changes

• Create and validate __episodes, __tasks, __info, and __stats companion tables for direct capture.
• Compute native LeRobot episode statistics when an episode is accepted and incrementally maintain global statistics across commits.
• Aggregate each feature using its declared dtype, avoiding LeRobot 0.4.4's image-name heuristic for ordinary numeric fields.
• Store task labels once in the tasks table while frame rows retain task_index.
• Optionally persist an immutable ordered vocabulary in __subtasks, validate frame-level subtask_index values, and restore the vocabulary on resume.
• Resume frame, episode, task, subtask, and statistics state from snapshot properties and projected companion metadata without scanning frame data.
• Commit companion metadata before frames and reject inconsistent or legacy frame-only table groups instead of silently continuing from partial state.
• Document the direct-capture table group, statistics, subtasks, resume behavior, and multi-table commit boundary.

Testing

• Run all LeRobot loader and writer unit tests (77 tests).
• Verify numeric and image episode/global statistics against native LeRobot implementations.
• Verify a numeric observation.image_embedding feature across multiple episodes and resume.
• Verify multiple flushes, resume, schema validation, task deduplication, subtask vocabulary persistence, and inconsistent companion state handling.
• Simulate an environment without the LeRobot extra: dependency-requiring tests skip while the missing-dependency test still runs.
• Run Ruff on all changed Python files.
• Run git diff --check.

Notes

• Direct capture stores image frames as PNG blobs; video remains unsupported.
• A new table declaring subtask_index requires an ordered, unique subtasks vocabulary. Existing groups restore it from __subtasks and reject conflicting vocabularies.
• Existing frame-only writer tables are not implicitly migrated because they do not contain the metadata required to reconstruct complete native LeRobot recording state.
• Paimon does not provide a transaction across the table group, so reopening validates component state and rejects a partial batch.

[JingsongLi]

Two regressions found in the direct writer changes: optional-dependency handling in the test matrix and aggregation of numeric features whose names contain image.

[JingsongLi]

The previous two findings are fixed. One integration issue remains with the PaimonLeRobotDataset reader now merged into master in #9498.

[JingsongLi]

[P1] Preserve the text index metadata for tasks and subtasks

These companion tables store labels as ordinary columns without pandas index metadata, but the PaimonLeRobotDataset reader merged in #9498 restores a DataFrame and uses its index as the task/subtask text (_component_dataframe and _index_names). Consequently, a recording with task="pick" is decoded as task "0", and its first frame fails _validate_control_row with Paimon task at LeRobot index 0 is not assigned to Episode 0, because the episode correctly lists "pick". Subtask labels are likewise decoded as numeric strings. I reproduced this using the PR writer's persisted output and the current master reader; restoring the text indexes makes the same frame pass validation. Please preserve the native text-index pandas metadata for both companion schemas using the existing _prepare_metadata_tables mechanism, or make the reader explicitly support these plain text columns, and add a writer-to-reader regression test.