Fix review comments: type hints, error handling, and documentation

[Copilot]

Addresses 8 review comments from PR #148 covering type annotations, error handling patterns, and documentation consistency.

Changes

chebai/preprocessing/migration/migrate_checkpoints.py

• Fixed argument count validation (< 2 → < 3 for two required arguments)
• Corrected script name in usage message (modify_checkpoints.py → migrate_checkpoints.py)
• Fixed docstring typo

chebai/result/prediction.py

• Replaced unnecessary try-except with None check after dict.get()
• Fixed typo: "piplines" → "pipelines"

chebai/preprocessing/datasets/base.py

• Updated model_hparams type documentation from Optional[dict] to dict to match implementation
• Added type annotation: idx → idx: int

README.md

• Fixed logger name inconsistency in example command (chebi50_bce_unweighted → chebi50_bce_weighted to match bce_weighted.yml criterion)

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