fix(data): validate explicit split columns

[DevChiniwala]

Problem

Explicit validation or test datasets that omit a training target or feature pass Phase 2 preparation. The schema check removed when explicit split support was added in #189 was left as a TODO, so invalid data reaches later workflow stages before producing a useful error.

Root cause

prepare_data materializes explicit splits but does not check that they contain the training columns before splitting or sampling.

Fix

Validate each explicit split against the training column names immediately after materialization. Raise a ValueError that identifies the split and missing columns. Split-only extra columns remain accepted.

Regression coverage

The deterministic unit test covers missing targets and features in both explicit validation and test splits, and asserts validation occurs before downstream splitting or sampling. Existing explicit-split coverage verifies extra columns remain accepted.

Validation

• pytest tests/unit/ — 113 passed, 14 skipped (Python 3.12 project environment)
• black --check . — passed
• ruff check . — passed
• git diff --check — passed
• Staged integration suite not run; it requires live provider credentials.

Follow-up to the explicit split workflow in #189.

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