Fix symbol extraction timeout isolation

[Widthdom]

Summary

• Route symbol extraction through a reusable isolated worker process so extraction timeouts kill the timed-out worker instead of leaving extractor work running in-process.
• Support framework-dependent, apphost, and single-file/self-contained worker launch paths for symbol extraction and post-extraction hook callbacks.
• Capture extractor console output inside the worker so stdout cannot corrupt the worker JSON protocol, while stderr diagnostics are forwarded after response parsing.
• Use source-generated worker JSON contexts so trimmed single-file binaries can serialize worker protocol messages without reflection metadata.
• Add focused regression coverage and a changelog fragment for full-scan symbol extraction timeouts should ensure extractor tasks stop #3048.

Issues

Fixes #3048

#3047 was already fixed on the fetched origin/main before this branch. I verified the existing hook callback timeout coverage and closed #3047 separately; this PR does not add a separate #3047 fix.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~IndexCommandRunnerTests.PostExtractionHookCallbackWorker_StartInfo_UsesCurrentCdidxExecutableWhenAvailable|FullyQualifiedName~IndexCommandRunnerTests.PostExtractionHookCallbackWorker_StartInfo_UsesFrameworkDependentDllWhenProcessIsNotCdidx|FullyQualifiedName~IndexCommandRunnerTests.SymbolExtractionWorker_StartInfo_UsesCurrentCdidxExecutableWhenAvailable|FullyQualifiedName~IndexCommandRunnerTests.SymbolExtractionWorker_StartInfo_UsesFrameworkDependentDllWhenProcessIsNotCdidx|FullyQualifiedName~IndexCommandRunnerTests.Run_PublishedSingleFileBinary_IndexesWithIsolatedSymbolWorker|FullyQualifiedName~PostExtractionHookTests" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~IndexCommandRunnerTests.Run_PublishedSingleFileBinary_IndexesWithIsolatedSymbolWorker|FullyQualifiedName~IndexCommandRunnerTests.Run_FullScanJson_WritesLivenessToStderrOnly" -p:UseSharedCompilation=false
• dotnet publish src/CodeIndex/CodeIndex.csproj --configuration Debug --runtime osx-arm64 --output /private/tmp/cdidx-singlefile-ci-check -p:PublishTrimmed=true -p:SelfContained=true -p:PublishSingleFile=true -p:UseSharedCompilation=false
• /private/tmp/cdidx-singlefile-ci-check/cdidx-smoke /private/tmp/cdidx-smoke-project --db /private/tmp/cdidx-smoke-db-sourcegen/index.db --json --force
• /private/tmp/cdidx-singlefile-ci-check/cdidx-smoke /private/tmp/cdidx-smoke-project --db /private/tmp/cdidx-smoke-db-sourcegen/stdout-stderr.db --json --force >/private/tmp/cdidx-smoke-stdout.json 2>/private/tmp/cdidx-smoke-stderr.txt
• dotnet build src/CodeIndex/CodeIndex.csproj -c Debug -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• git diff --check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --files src/CodeIndex/Indexer/Symbols/SymbolExtractionWorker.cs src/CodeIndex/Indexer/Hooks/PostExtractionHookCallbackWorker.cs tests/CodeIndex.Tests/IndexCommandRunnerTests.cs --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits HEAD --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.