fix: force-kill the captured PET child so restart cannot lose or misfire the kill

[StellaHuang95]

Fixes a force-kill ownership bug in the PET (python-environment-tools) child-process teardown in src/managers/common/nativePythonFinder.ts.

Problem

PET runs as a long-lived child process managed by NativePythonFinderImpl. When a request times out or the RPC connection breaks, recovery calls killProcess() to terminate the hung child, then restart() spawns a fresh one into this.proc. The teardown could act on the wrong process across that restart boundary.

Root cause

killProcess() sent SIGTERM, scheduled a 500 ms setTimeout that re-read the mutable this.proc field to decide whether to SIGKILL, then synchronously set this.proc = undefined. When the callback ran, this.proc was either already undefined (so a hung PET was never SIGKILLed) or had been reassigned by a concurrent restart (so the callback could SIGKILL the healthy replacement).

Fix

Extract killPetProcessWithGrace(getProc, clearProc, outputChannel, graceMs), which captures the current child and clears the holder before signalling, then sends SIGTERM and — after the grace period — SIGKILLs only that captured child. killProcess() delegates via () => this.proc / () => { this.proc = undefined; }. Because the delayed SIGKILL closes over the captured local child rather than the mutable field, a concurrent restart's replacement is never targeted, and a hung child is reliably force-killed. The grace period is unchanged (KILL_PROCESS_GRACE_PERIOD_MS = 500). No PET source is touched and there is no extension-API change.

Tests

nativePythonFinder.killProcess.unit.test.ts (6 cases, sinon fake timers): SIGTERM→SIGKILL on the captured child, synchronous ownership release, exit-during-grace not force-killed, already-exited child not signalled (holder still cleared), a replacement assigned during the grace period is never killed (key regression), and SIGTERM errors are caught/logged without throwing. All 6 pass; npm run lint and npm run compile-tests are clean.

[StellaHuang95]

🔒 Automated review in progress — @StellaHuang95 is auto-reviewing this PR.