Support CSV and .xlsm inputs in generator and add CSV parsing/tests

[0x5143]

Motivation

• Extend the generator to accept additional input formats (.csv and .xlsm) so tables can be authored as CSV or macro-enabled workbooks and processed by the same pipeline.

Description

• Update documentation to mention .xlsm and .csv input support and add the new patterns to example dotnet dtgen invocations and README text.
• Add IsSupportedInputFile and restrict file enumeration to formats the generator actually supports (.xlsx, .xlsm, .csv).
• Introduce CreateWorkbook, GetCsvSheetName, and ReadCsvRows to map a CSV file into a single in-memory XSSFWorkbook worksheet and reuse the existing workbook parsing logic.
• Replace direct new XSSFWorkbook(stream) usage with a call to CreateWorkbook in GenerateExcel, and add InputFormatGenerationTests unit tests covering CSV and .xlsm inputs.

Testing

• Ran InputFormatGenerationTests.CsvInput_ShouldGenerateCodeAndData which created a CSV, invoked the generator, and asserted generated DRItem.cs and DRItem.bytes, and the test passed.
• Ran InputFormatGenerationTests.XlsmInput_ShouldGenerateCodeAndData which created an .xlsm workbook, invoked the generator, and asserted generated DRItem.cs and DRItem.bytes, and the test passed.

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Codex Task