test(integration): raise integration coverage above 80%

[samtrion]

Summary

The integration test project only covered about 21% of lines / 22% of branches, well below the target of >80%. The unit test suite was already well above target, but the combined coverage requirement (>90%) was also at risk because of the integration project's low numbers.

Changes

Added integration tests that exercise the previously untested parts of the public API through realistic code-generation scenarios:

• CSharpCodeBuilderTests.ConditionalApi.cs — every AppendIf/AppendLineIf overload, both the true and false branch, including the pointer-based overload
• CSharpCodeBuilderTests.AppendVariants.cs — char[], ReadOnlyMemory<char>, ReadOnlySpan<char>, and pointer-based Append sources, plus special-character handling ({, }, \n, \0)
• CSharpCodeBuilderTests.DocumentationApi.cs — the full AppendXmlDoc* family, including null/empty inputs
• CSharpCodeBuilderTests.FormatAndInterpolationApi.cs — all AppendFormat/AppendLineFormat overloads and the AppendInterpolated/AppendLineInterpolated interpolated string handler
• CSharpCodeBuilderTests.ScopeAndCapacityApi.cs — nested Scope()/ScopeLine() usage, Clear(), Indent()/Intend(), EnsureCapacity()

Coverage (measured locally)

Suite	Line	Branch
Unit	90.4%	86.8%
Integration	88.3%	83.5%
Combined	93.6%	90.5%

All targets (>80% unit, >80% integration, >90% combined) are met.

Testing

• dotnet test — 395 unit tests and 27 integration tests, all passing
• Coverage collected via Microsoft.Testing.Extensions.CodeCoverage (cobertura) and merged with reportgenerator

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.76%. Comparing base (1f0fe4d) to head (06a9894).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #300      +/-   ##
==========================================
+ Coverage   85.00%   91.76%   +6.76%     
==========================================
  Files          15       15              
  Lines         340      340              
  Branches      100      100              
==========================================
+ Hits          289      312      +23     
+ Misses         33       17      -16     
+ Partials       18       11       -7     

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