Fix C# raw-string multiline field extraction

[Widthdom]

Summary

• bound the expensive C# multiline declaration confirmation phase without truncating confirmed field signatures
• preserve full signatures for wrapped static readonly raw-string fields, same-line const raw-string fields, and long object/collection initializers
• add extractor-level and CLI-level regression coverage for symbols / outline output so signature truncation is caught end-to-end

Root Cause

The earlier #447 perf fix capped multiline lookahead, but confirmed field declarations still depended on partial candidate assembly when building stored signatures. That left long raw-string declarations, especially same-line const fields, truncated at the opener line even though the symbol itself still existed.

Impact

symbols, definition, and outline now retain the full declaration signature for long multiline C# fields instead of stopping at string or """.

Validation

• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SymbolExtractorTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~RunSymbolsAndOutline_CSharpLongRawStringFields_PreserveFullSignature"
• local built dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll on temporary projects to verify symbols and outline output for long raw-string fields

Closes #447.