[codex] Fix C va_arg typedef reference extraction

[Widthdom]

Summary

• Parse C va_arg / __builtin_va_arg type operands from balanced call arguments.
• Use a C-specific argument splitter so nested commas and < comparisons in the first operand do not hide the real type operand.
• Add regression coverage and a changelog fragment for net9 Release test can miss lowercase va_arg typedef reference #2961.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --filter "FullyQualifiedName~ReferenceExtractorTests.Extract_CTypedefVaArgOperands" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --filter "FullyQualifiedName~ReferenceExtractorTests.Extract_CTypedefVaArgOperands" -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• git diff --check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Known unrelated full-suite flake is already tracked in #2945.

AGENT_GUIDE.md, AGENTS.md/AGENT.md, and CLAUDE.md were not modified.

Fixes #2961