🧪 [testing improvement description] Add test for schema validation dtype mismatch

[edithatogo]

• 🎯 What: The testing gap addressed is the missing test for validate_schema when a column has a different dtype than the expected schema.
• 📊 Coverage: The scenario where validate_schema receives a mismatched column type (e.g., Int64 instead of String) is now tested. It validates that the correct CoreError::TransformationError is thrown, and that the error message correctly describes the type issue.
• ✨ Result: Improved test coverage for validate_schema in src/pipeline.rs, ensuring it handles mismatching schema definitions correctly.

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PR created automatically by Jules for task 13594991189111247669 started by @edithatogo

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