fix: drain process output before returning, kill orphaned speedtest, thread-safe capture

[laurentiu021]

External-process plumbing defects across the shared runner and two consumers.

Changes

• PowerShellRunner.RunProcessAsync — WaitForExitAsync returns when the process exits, but the async BeginOutputReadLine/BeginErrorReadLine pumps may not have raised their final lines, so callers that snapshot output immediately lost the last lines (e.g. NetworkRepair, chkdsk, winget). Added a proc.WaitForExit() (on a thread-pool thread, only when not cancelled) to block until both readers reach end-of-stream. Fixes the truncation at the seam for all consumers.
• SpeedTestService.RunOoklaAsync — cancellation/timeout during the parallel stdout/stderr ReadToEndAsync propagated before the try/catch that kills the child, orphaning speedtest.exe. Wrapped the reads + wait in one try; the OperationCanceledException catch now kills the process tree on any cancellation path.
• SystemHealthViewModel (chkdsk) and WingetService (upgrade scan) — both collected output into a plain List<string> written by the concurrent stdout+stderr reader threads (data race: dropped/corrupted lines). Switched to ConcurrentQueue<string>, materialized (ToArray()/ToList()) before parsing.

Tests

• Existing WingetService/SystemHealth parse tests still pass (the parsers are unchanged; only the collector type changed). The concurrency/orphan-kill paths require live process I/O and aren't unit-testable; they mirror the audit's recommended fixes.

Build: 0 warnings / 0 errors. Version 1.20.38.