fix: nchar convert to decimal error

[facetosea]

Description

Please briefly describe the code changes in this pull request.

Checklist

Please check the items in the checklist if applicable.

• Is the user manual updated?
• Are the test cases passed and automated?
• Is there no significant decrease in test coverage?

[gemini-code-assist]

Summary of Changes

Hello @facetosea, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a bug in the decimal conversion logic where string-based data types like NCHAR, VARCHAR, and VARBINARY were not being correctly converted to DECIMAL due to an erroneous length calculation. The fix involves adjusting the length parameter passed during the conversion. Additionally, comprehensive test cases have been added to ensure the robustness and accuracy of decimal-to-NCHAR conversions across different precision and scale settings.

Highlights

• Decimal Conversion Fix: Corrected an error in the decimal##FromStr macro where VARSTR_HEADER_SIZE was incorrectly subtracted from the input string length for NCHAR, VARCHAR, and VARBINARY types, leading to potential conversion issues when casting these types to DECIMAL.
• New Test Cases: Introduced a new test method constValToDecimal in test_datatype_decimal.py to thoroughly validate the conversion of constant decimal values to NCHAR, covering various precision, scale, and rounding scenarios.

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[gemini-code-assist]

Code Review

This pull request aims to fix an error when converting NCHAR data to DECIMAL. The fix in decimal.c adjusts the string length used in the conversion, which seems correct for NCHAR. However, this change also affects VARCHAR and VARBINARY types, and it's important to verify that this is intended and does not cause regressions. I've added a comment regarding this. The Python test file introduces new tests, but they seem to test DECIMAL to NCHAR conversion, not the NCHAR to DECIMAL path that was fixed. I've provided suggestions to add a more appropriate test case to validate the fix. Additionally, I found a duplicated test case that should be removed for better maintainability.