[python] Add LeRobot Dataset v3 import

[XiaoHongbo-Hope]

Purpose

Add a one-time image-based LeRobot v3 importer:

LeRobot Dataset v3 -> Arrow batches -> Paimon table

snapshot_id = conn.load_from_lerobot(
    "robot_data",
    "/path/to/lerobot_dataset",
)

• Supports local directories, FileIO URIs such as OSS through source_options, and Hugging Face repo_id.
• Derives the schema from meta/info.json, preserves feature names and Episode boundaries, writes Arrow batches, and creates one snapshot.
• Maps scalars to scalar types, vectors to VECTOR, higher-rank tensors to nested ARRAY, and compressed images to BLOB.
• Reuses the HDF5 strict existing-table schema validation and append semantics.
• Keeps LeRobot optional through pypaimon[lerobot] and rejects v2.x clearly.

This PR supports image features only. Video features are rejected with a clear error; packed video import will be a separate follow-up on #9473. This PR does not depend on #9473.

Tests

• Official LeRobot API creates a two-Episode v3 image dataset in a temporary directory without network access.
• Covers metadata-derived schema, scalar/vector/tensor values, ordering, image BLOB round trip, append, snapshot, incompatible schema, OSS reads, missing dependency, v2 rejection, and video rejection.
• multimodal_lerobot_test.py: 7 passed
• multimodal_hdf5_test.py: 24 passed
• Flake8, py_compile, and git diff --check: passed