Added AllowFieldCountMismatch property to CsvConfiguration

[michelebastione]

The implementation of this property makes jagged rows being returned as they are instead of having an exception being thrown when a mismatched number of fields is found.

Resolves #979

Summary by CodeRabbit

• New Features

  • Added an option to interpret empty CSV fields as default values.
  • Added support for reading rows with varying numbers of fields when field-count mismatches are allowed.
  • Improved handling of missing and additional columns during CSV import.

• Bug Fixes

  • Corrected reported column names for missing-column errors.

• Deprecation

  • Replaced the previous empty-string handling option with the new configuration setting.

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📝 Walkthrough

Walkthrough

CsvReader.QueryAsync now handles empty fields and variable-width rows through the new ReadEmptyFieldsAsDefault option and existing mismatch configuration. Tests cover headered, headerless, and mapped jagged CSV rows.

Changes

CSV jagged-row handling

Layer / File(s)	Summary
Empty-field configuration contract src/MiniExcel.Csv/CsvConfiguration.cs	Adds ReadEmptyFieldsAsDefault and makes obsolete ReadEmptyStringAsNull delegate to it.
Row parsing and result mapping src/MiniExcel.Csv/CsvReader.cs	Refactors row parsing, mismatch validation, synthetic columns, filler fields, and empty-field conversion for headered and headerless CSV data.
Jagged-row and exception coverage tests/MiniExcel.Csv.Tests/Main/MiniExcelCsvAsyncTests.cs, tests/MiniExcel.Csv.Tests/Main/MiniExcelCsvTests.cs, tests/MiniExcel.Csv.Tests/Main/Models.cs	Adds jagged-row tests and DTO fields, and updates column-name and dictionary assertions.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly names the main change: adding CsvConfiguration support for mismatched CSV field counts.
Linked Issues check	✅ Passed	The changes add AllowFieldCountMismatch, update CSV parsing to tolerate jagged rows, and add tests matching issue #979.
Out of Scope Changes check	✅ Passed	The extra CSV empty-field and test/model updates appear to support the same jagged-row parsing feature and are not unrelated.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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Actionable comments posted: 1

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Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/MiniExcel.Csv/CsvReader.cs`:
- Around line 67-133: Update the invalid-row check before the header and
headerless branches to reject any field-count mismatch when
AllowFieldCountMismatch is false, not only rows where fields.Length is less than
headerRow.Count. Preserve the existing ColumnNotFoundException construction and
ensure both hasHeaderRow paths exit through this check before assigning fields.

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Fix all unresolved CodeRabbit comments on this PR:

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ℹ️ Review info

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Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between 3266e8b and 87445aa.

📒 Files selected for processing (5)

• src/MiniExcel.Csv/CsvConfiguration.cs
• src/MiniExcel.Csv/CsvReader.cs
• tests/MiniExcel.Csv.Tests/Main/MiniExcelCsvAsyncTests.cs
• tests/MiniExcel.Csv.Tests/Main/MiniExcelCsvTests.cs
• tests/MiniExcel.Csv.Tests/Main/Models.cs