ci(windows): make taskkill no-op when server.py already exited

The path-discovery step succeeds on the first run, but the cleanup
step exits non-zero because `taskkill /PID 5560 /T /F` returns 128
("process not found") when server.py has already exited on the mock
hermes_cli stub. That's the expected steady state for this mock-only
workflow, not a failure.

Two-line fix: reset `$global:LASTEXITCODE = 0` after the taskkill
call, and explicit `exit 0` at the end of the step so any other
external-command exit codes don't bubble up. The try/catch wrapper
didn't help because taskkill writes its diagnostic to stderr without
raising a PowerShell exception — `catch` never fired.

Run 26352805510 on this branch shows the failure shape: "OK: start.ps1
path discovery - all guards passed." in the verify step, then
"ERROR: The process '5560' not found." in the cleanup step. Path
discovery is what this workflow exists to validate; cleanup just has
to not fail the job.

.github/workflows/native-windows-startup.yml

@@ -109,19 +109,18 @@ jobs:
           }
           Write-Host "OK: start.ps1 path discovery - all guards passed."
 
-      # taskkill /T walks the process tree, /F forces. taskkill exits
-      # non-zero if the PID is already gone (server.py crashed on the
-      # stub) — that's expected, not a failure.
+      # taskkill /T walks the process tree, /F forces. taskkill returns
+      # 128 ("process not found") if the PID is already gone — that's
+      # the expected steady state for this mock-only workflow because
+      # server.py exits immediately on the stub hermes_cli. Reset
+      # $LASTEXITCODE so the step never fails on the cleanup itself.
       - name: Stop background server (tree-kill)
         if: always()
         shell: pwsh
         run: |
           if ($env:SERVER_PID) {
-            try {
-              & taskkill /PID $env:SERVER_PID /T /F 2>&1 | Out-Host
-            } catch {
-              Write-Host "taskkill: PID $env:SERVER_PID already exited (expected with mock stub)"
-            }
+            & taskkill /PID $env:SERVER_PID /T /F 2>&1 | Out-Host
+            $global:LASTEXITCODE = 0
           }
           # Belt-and-suspenders: kill anything still bound to 8787.
           $hanging = Get-NetTCPConnection -LocalPort 8787 -State Listen -ErrorAction SilentlyContinue
@@ -130,3 +129,4 @@ jobs:
               try { Stop-Process -Id $c.OwningProcess -Force -ErrorAction Stop } catch {}
             }
           }
+          exit 0