[codex] Split symbol and reference extraction modules

[Widthdom]

Summary

• Split C# symbol pattern constants into a dedicated partial module for SymbolExtractor language patterns should be split further by language or concern #3381.
• Moved Lua reference extraction helpers from the shared support class into the Lua reference extractor for Reference extraction support should be split into smaller language and helper modules #3421.
• Split Lua symbol extractor coverage out of the mega SymbolExtractorTests file for Mega test files should be split to improve targeted validation and review #3383.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter FullyQualifiedName~SymbolExtractorTests
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter FullyQualifiedName~ReferenceExtractorLuaTests
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~ReferenceExtractorTests&FullyQualifiedName~Lua"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: no blocking/actionable issues found.

Documentation / Changelog

• Added changelog.d/unreleased/3381.internal.md
• Added changelog.d/unreleased/3421.internal.md
• Added changelog.d/unreleased/3383.internal.md

Follow-up Candidates

None.

Fixes #3381
Fixes #3421
Fixes #3383