full-scan symbol extraction timeouts should ensure extractor tasks stop

[Widthdom]

Problem

The full-scan path starts symbol extraction in a task, cancels the linked token after a timeout, waits one additional second, and then throws even if the extraction task is still running.

Evidence:

• src/CodeIndex/Cli/IndexCommandRunner.FullScan.cs:116
• src/CodeIndex/Cli/IndexCommandRunner.FullScan.cs:133
• src/CodeIndex/Cli/IndexCommandRunner.FullScan.cs:145

A non-cooperative extractor can continue consuming CPU or holding resources after the index command has already failed.

Suggested fix

Ensure extractors observe cancellation promptly, wait for a confirmed stop before returning failure, or isolate timeout-bounded extraction so abandoned work cannot continue in the process. Add a regression test with a deliberately non-cooperative extractor.

[Widthdom]

Work started on branch fix-issue3048-3047.

[Widthdom]

対応PRを作成しました: #3328

#3048 の対応として、symbol extraction を隔離 worker process 経由に変更し、timeout 時に対象 worker を終了させる形にしています。