fix(win): don't close the pseudoconsole twice on a second kill()

[shrmguy (leroyomey)]

Fixes #983.

PtyKill closes the pseudoconsole but leaves handle->hpc pointing at it, and the handle stays in ptyHandles until the exit watcher removes it. A second kill() in that gap closes the same pseudoconsole again, which with useConptyDll: true takes the process down with 0xC0000374.

PtyKill now sets hpc to null once it has closed it, and skips the close if it's already null. I added the same check to PtyResize and PtyClear, since they can run in the same gap and were passing the closed handle along.

The script from the issue crashed every time for me on Windows 11 (26200, Node 24) and survives with this change, with cmd.exe and powershell.exe. The new test takes down the mocha run without the fix. npm test passes.

My Build Tools install has no Spectre libraries, so I built locally with SpectreMitigation off. I didn't try the onExit timing variant from the issue, it needs a loaded machine. It goes through the same second close, so this should cover it.