Test refactorings for improved code coverage

[michelebastione]

The following changes have been implemented to improve test coverage:

• Added .runsettings file for code coverage configuration
• Expanded test target frameworks to include NET8 and NET9
• Separated OpenXmlImporter and OpenXmlExporter tests
• Added new tests for retrieving sheet informations, sheet dimensions and column names
• Added new queryrange tests
• Removed obsolete FileHelper test utility class
• Removed removed superfluous excel test files

closes #864

[gemini-code-assist]

Code Review

This pull request introduces several enhancements and test refactorings, including adding a .runsettings file for code coverage, making NextResult abstract in MiniExcelDataReaderBase, adding .xlsm format validation, and splitting OpenXml importer and exporter tests into dedicated files. Feedback on the changes highlights a redundant stream in FillTemplateAsync, potential file locking issues in template tests due to stream scope, and compiler warnings from synchronous test methods incorrectly marked as async Task. Additionally, it is recommended to use StringComparison.OrdinalIgnoreCase for file extension comparisons.

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