Add tests for image deletion and fix mixed image-video deletion

[atyshka]

What this does

Currently, if you have a dataset with both image and video features and batch_encoding_size > 1, the tmp images are deleted for both image features and video features at the end of every episode. These images are then missing when batch video encoding is triggered. This fix ensures that only image features are deleted at the end of an episode.

How it was tested

Added 4 new tests to test_datasets.py testing image deletion for image features, unbatched video, batched video, and mixed datasets.

PLEASE NOTE: the test for batched video is currently failing due to #2404 and requires the fix in #2462

How to checkout & try? (for the reviewer)

python -m pytest tests/datasets/test_datasets.py

[Copilot]

Pull request overview

This PR fixes a critical bug in temporary image cleanup that was breaking batch video encoding for mixed image-video datasets. The fix ensures that only image features have their temporary directories deleted after each episode, while video features retain their temporary images until batch encoding is triggered.

• Corrects clear_episode_buffer() to only delete temporary images for image features (dtype="image"), not all camera keys
• Adds comprehensive test coverage for image deletion behavior across different scenarios (image-only, video with batch_encoding_size=1, video with batch_encoding_size>1, and mixed datasets)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/lerobot/datasets/lerobot_dataset.py	Changes clear_episode_buffer() to use image_keys instead of camera_keys when deleting temporary image directories, preventing premature deletion of video feature temporary images
tests/datasets/test_datasets.py	Adds 4 new test functions to verify correct temporary image deletion behavior for different feature types and batch encoding settings

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[atyshka]

@CarolinePascal This is an update to #1751 post-dataset-v3 if you have a chance to review

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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