Fix C# graph and LSP common-token conflation

[Widthdom]

Summary

• Prevent C# argument-list member calls from being indexed as standalone function definitions and keep bare exact C# caller graph queries from counting receiver-qualified calls.
• Scope LSP definition and references to the resolved indexed document before workspace fallback, including common tokens with no indexed definition target.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug --filter "FullyQualifiedName~SymbolExtractorTests.Extract_CsharpExpressionArgumentMemberCalls_AreNotDefinitions|FullyQualifiedName~DbReaderTests.GraphQueries_CsharpBareMemberCallersSkipReceiverQualifiedCalls|FullyQualifiedName~LspServerTests.HandleMessage_Definition_PrefersCurrentIndexedDocumentForCommonToken|FullyQualifiedName~LspServerTests.HandleMessage_References_PrefersCurrentIndexedDocumentForCommonToken|FullyQualifiedName~LspServerTests.HandleMessage_References_PrefersCurrentIndexedDocumentWhenCommonTokenHasNoDefinitions"
• 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 . --changed-between origin/main HEAD --json

Documentation And Changelog

• Added changelog.d/unreleased/2818.fixed.md.
• Added changelog.d/unreleased/2831.fixed.md.

Review

No blocking/actionable issues found.

Follow-ups

None.

Fixes #2818
Fixes #2831