Fix Python dynamic import indexing

[Widthdom]

Summary

• Index literal Python dynamic imports from importlib.import_module, importlib.util.find_spec, and __import__.
• Emit reference entries for literal dynamic imports while avoiding comments, string literals, and variable module names.
• Add regression coverage for symbol extraction and reference extraction.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~SymbolExtractorTests.Extract_Python_IndexesDynamicImportLiteralModules|FullyQualifiedName~ReferenceExtractorTests.Extract_PythonDynamicImports_EmitImportAndImportlibReferences"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~SymbolExtractorTests.Extract_Python|FullyQualifiedName~ReferenceExtractorTests.Extract_Python"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits ce49d3efe1ae42cef0e0ff92e3cbc64a1677f167 --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• Added changelog fragment: changelog.d/unreleased/2056.fixed.md

Follow-up Candidates

• None.

Fixes #2056