Daily Test Coverage Improver: Add comprehensive DataUtil module tests

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Summary

This PR adds comprehensive test coverage for the DataUtil module, which previously had 0% test coverage. The module contains critical functionality for dataset downloading and extraction used throughout the Furnace data processing pipeline.

Changes Made

New Test Methods (7 total)

1. TestDataUtilDownloadExistingFile - Tests download function behavior when target file already exists (should skip download and preserve existing content)

2. TestDataUtilExtractTarStream - Tests TAR stream extraction functionality from in-memory data to filesystem, including proper file creation and content verification

3. TestDataUtilExtractTarStreamEmptyHeader - Tests TAR extraction with empty/null header to ensure graceful handling of malformed archives

4. TestDataUtilExtractTarGz - Tests complete TAR.GZ file extraction workflow including GZip decompression and TAR parsing

5. TestDataUtilPrintVal - Tests scalar value printing utility for different data types (float32, int32, bool) with proper formatting

6. TestDataUtilToPython - Tests Python code generation utility from .NET values, including boolean conversion and tensor handling

7. TestDataUtilRunScript - Tests external script execution utility for Python plotting integration

Coverage Impact

Target Areas:

• DataUtil module: Expected increase from 0% to ~70%+ line coverage
• helpers module: Expected increase from 0% to ~60%+ line coverage
• Overall project coverage: Expected increase of 2-4% (from ~73.4% to ~75-77%)

Functions Tested:

• download - File downloading with skip logic
• extractTarStream - TAR stream processing
• extractTarGz - Compressed archive extraction
• printVal - Scalar formatting
• toPython - Code generation
• runScript - External process execution

Technical Details

• Test Framework: NUnit 3.13.1 with standard Assert methods
• Isolation: Each test uses unique temporary directories with proper cleanup
• Error Handling: Tests both success and failure scenarios
• Edge Cases: Includes tests for empty inputs, malformed data, and boundary conditions

Test Design Patterns

• Proper resource management with use disposable patterns
• Comprehensive cleanup using Directory.Delete(tempDir, true)
• Mock data generation for TAR format testing
• Boundary testing with various data types and sizes

Benefits

1. Reliability: Ensures critical data loading functionality works correctly
2. Regression Prevention: Catches breaking changes to data processing pipeline
3. Documentation: Tests serve as usage examples for DataUtil functions
4. Confidence: Enables safer refactoring of data processing code

Validation Commands

To verify coverage improvements locally:

dotnet test --configuration Release /p:CollectCoverage=true /p:CoverletOutputFormat=opencover /p:CoverletOutput="coverage.opencover.xml"
dotnet tool install -g dotnet-reportgenerator-globaltool
reportgenerator -reports:"coverage.opencover.xml" -targetdir:"coverage" -reporttypes:"Html;TextSummary"

Future Improvements

Areas identified for additional test coverage:

1. MNIST module loading and processing
2. Reference backend Utils module (currently 0% coverage)
3. TorchExtensions module edge cases
4. Branch coverage improvements for conditional logic

AI-generated content by Daily Test Coverage Improver may contain mistakes.