added tests trasnforms and scripts for v2 and added event.copy() at tranforms v2 EnglishBCBLPreprocessing._run mutates events

[Karan24Soni]

Summary

v2 is missing test coverage that v1 already has. v1 has tests for transforms,
scripts, model and pl_module. v2 only had model and losses tests, nothing for
transforms.py or scripts/extract_predictions.py, even though those are the
most likely place for silent breakage (pandas event preprocessing, the
splitter, the CLI metrics script).

Added tests/test_v2_transforms.py covering EnglishBCBLPreprocessing,
Brain2QwertyV2Splitter, WordCreator and SentenceKeySeq - practice trial
removal, space -> "&" normalization, dropping / tokens and
any out-of-vocab key, perception trials and too-sparse sentences getting
dropped, button_UID ordering, and split determinism / no leakage across a
sentence's rows.

Added tests/test_v2_scripts.py covering extract_predictions.py
(compute_cer, compute_wer, summarize, print_summary, main) including the
CSV path resolution and the --output summary file.

Also fixed a SettingWithCopyWarning in EnglishBCBLPreprocessing._run -
events wasn't copied after the first filter, unlike the rest of the
pipeline. Doesn't change any computed values, just stops the warning.

Test plan

• pytest passes locally
• ruff check and ruff format --check pass

Checklist

• I have signed the Meta CLA
• My changes follow the contributing guide

[Karan24Soni]

@jarod1212 could you check this out if possible