test: add CLI test suite and test datasets

[mitsuki31]

Overview

This PR introduces an initial testing setup for the CLI, including dataset fixtures and development dependencies. It establishes a foundation for validating functionality and preventing regressions.

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Changes Made

Test Suite

• Added initial test module for CLI using pytest and CliRunner
• Covers basic command execution and file conversion flows

Test Data

• Added Excel datasets under tests/data for various scenarios:
  • basic data
  • multi-sheet
  • empty (including NULL values) and header-only files
  • financial data
  • special characters and encoding
  • large numbers and long text

Note

Inside test suite module, there's a test function which converts an empty Excel file into CSV, but the test itself will returns success due to the Excel file having a sheet. The tool will only fails if the Excel file has no sheet at all, throwing xls2csv.exception.NoSheetFoundError exception.

Development Dependencies

• Added pytest as a development dependency in pyproject.toml
• Added pytest to requirements.txt

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Impact

• Enables automated testing for CLI behavior
• Improves reliability by covering common and edge-case inputs
• No impact on runtime users (dev dependency only)

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Summary

This PR sets up a basic but practical testing framework for the CLI, backed by realistic Excel datasets. It provides a starting point for expanding test coverage and improving code quality over time.