Add data validation gate and reporting

[AKKI0511]

Summary

• add reusable validation/reporting helper and enforce data checks across training, evaluation, and model backtests
• extend CLI commands with --skip-validation flag and emit validation reports in experiment/backtest folders
• add coverage for validation gating behavior and refresh documentation for the new workflow

Testing

• poetry run pytest
• poetry run pre-commit run --all-files

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Allow sparse optional columns in validation

Validation now computes nan_ratio_by_column across every column and then fails if the maximum exceeds 1%, which includes optional columns such as the news text column added in _attach_news. When news ingestion is enabled, that column is expected to be mostly null (no headline every bar), so max_nan_ratio will routinely exceed the 1% threshold and _validate_or_raise will halt training/backtesting even though the required OHLCV data is present. Validation should ignore optional/sparse fields or limit the NaN check to required price columns to avoid blocking any configuration that enables news ingestion.

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