Handle file deletion races during indexing

[Widthdom]

Summary

• Distinguish files that disappear during scan probes from unsupported/probe-failed files.
• Report deletion races as non-fatal warnings in scan/update/full-index paths and remove stale rows when possible.
• Add a focused FileIndexer regression test and a bilingual changelog fragment.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~FileIndexerTests" passed before the latest origin/main merge.
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~FileIndexerTests.ScanFilesDetailed_FileDeletedAfterEnumeration_RecordsWarning" passed after the latest origin/main merge.
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false passed after the latest origin/main merge.
• dotnet run --project tools/CodeIndex.Changelog -- check passed.
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false was attempted, but a trimmed publish test failed due an unrelated file lock on src/CodeIndex/obj/Debug/net8.0/osx-arm64/linked/cdidx.pdb while publishing.

Review

• Manual adversarial review of origin/main..HEAD: No blocking/actionable issues found.
• codex exec adversarial review was attempted but could not run because the local Codex CLI reported a usage limit.

Changelog

• changelog.d/unreleased/2044.fixed.md

Fixes #2044