Daily Test Coverage Improver: Add comprehensive JsonDocument and HTML runtime type tests

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Summary

This PR adds extensive test coverage for JsonDocument methods and HTML runtime record types that were previously at 0% coverage, following the systematic approach outlined in issue #1574.

Coverage Improvements Achieved

JsonDocument (FSharp.Data.Json.Core):

• Coverage improvement: 61.68% → 63.2% (+1.52 percentage points)
• 5 comprehensive test methods covering JsonDocument.Create overloads
• Tests JsonDocument parsing from TextReader for simple/complex JSON and arrays
• Tests JsonDocument.CreateList for handling JSON arrays and multiple objects
• Uses reflection to safely test methods marked "for generated code only"

HTML Runtime Types (FSharp.Data.Html.Core):

• Coverage improvement: 87.72% → 88.05% (+0.33 percentage points)
• 6 test methods for HtmlList and HtmlDefinitionList record types
• Tests record construction, property access, and ToString() formatting
• Covers edge cases like empty values and definitions lists

Technical Details

Implementation:

• New file: tests/FSharp.Data.Core.Tests/HtmlRuntimeTypes.fs - dedicated tests for HTML runtime records
• Enhanced: tests/FSharp.Data.Core.Tests/JsonRuntime.fs - added JsonDocument-specific coverage tests
• Project file updated: Added new test file to FSharp.Data.Core.Tests.fsproj
• All tests passing: 2723 tests with zero regressions

Coverage Strategy:

• Strategic focus on 0% coverage areas identified in systematic coverage analysis
• Reflection-based testing for internal/generated-code-only methods to avoid compiler warnings
• Comprehensive edge case testing (empty inputs, complex JSON structures, etc.)

Overall Impact

Project-wide metrics:

• Total project line coverage: 73.7% → 74.0% (+0.3 percentage points)
• Method coverage: 51.86% → 52.42% (+0.56 percentage points)
• Zero breaking changes: All existing functionality preserved

Test Plan

✅ Build successful: All projects compile without warnings
✅ All tests passing: 2723 tests execute successfully
✅ Code formatting clean: Fantomas formatting applied
✅ Coverage validated: Coverlet reports confirm improvements
✅ Zero regressions: Existing test suite unaffected

Commands to validate coverage improvements:

# Run tests with coverage collection
dotnet test tests/FSharp.Data.Core.Tests/FSharp.Data.Core.Tests.fsproj \
  /p:CollectCoverage=true \
  /p:CoverletOutputFormat=cobertura \
  /p:CoverletOutput=./TestResults/coverage.xml \
  --configuration Release

# Generate detailed coverage report  
reportgenerator \
  -reports:"tests/FSharp.Data.Core.Tests/TestResults/coverage.xml" \
  -targetdir:"TestResults/CoverageReport" \
  -reporttypes:"Html;TextSummary"

Future Work Areas

Based on this systematic approach, remaining 0% coverage opportunities include:

• FSharp.Data.Runtime.BaseTypes.XmlElement (Xml.Core)
• FSharp.Data.Runtime.XmlRuntime (Xml.Core)
• FSharp.Data.Runtime.XmlSchema (Xml.Core)
• Various WorldBank runtime classes

This PR demonstrates the effectiveness of the systematic test coverage improvement workflow outlined in the research issue.

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Detailed execution log

Bash commands executed:

• dotnet tool restore - Restored .NET tools
• dotnet paket restore - Restored Paket dependencies
• dotnet run --project build/build.fsproj -- -t Build - Built all projects
• dotnet test with coverage collection - Executed tests with metrics
• reportgenerator - Generated detailed coverage reports
• dotnet fantomas . - Applied code formatting
• git commands - Staged, committed, and pushed changes

Web searches performed: None

Web pages fetched: None

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