fix: stop host-mode jobs from leaking processes on Windows (#1080)

## Problem

On host-mode Windows runners, a job can leave processes running after it finishes,
and those leftovers hold file handles that block deletion of the workspace.

Today a step's tree is only torn down when the step is *cancelled*
(`process.Killer`). A step that completes leaves whatever it spawned alive, and
the existing workspace scan in `terminateRunningProcesses` misses two shapes of
leftover: orphans whose parent already exited (no tree to walk, and their
executable often lives outside the workspace), and processes that merely *run in*
the workspace but reference no path from it — `Win32_Process` exposes no working
directory, so the scan cannot match them.

## Solution

Two additions in `internal/pkg/process`, both no-ops outside Windows:

- **`process.Group`** — a job-scoped Windows Job Object created with
  `JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE`. Step processes are assigned to it before
  they get their own `Killer`, so the step's job nests inside the job's:
  cancellation still kills exactly the step's tree, while `Remove` closing the
  group makes the kernel terminate everything still assigned, whatever its
  parentage. The kernel also drops the handle when the runner exits, so a crashed
  runner cannot strand processes.

- **`process.KillProcessesWithCWDUnder`** — a best-effort net for processes that
  never joined the job (started via a service or scheduled task). It reads each
  process's working directory from its PEB and terminates those under a workspace
  dir. Processes it cannot open are skipped.

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1080
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

internal/pkg/process/leftover_windows_test.go

@@ -0,0 +1,91 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package process
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+	"golang.org/x/sys/windows"
+)
+
+// startSleeperIn launches a long-running child whose cwd is dir but whose
+// executable lives outside it — what the path scan cannot see.
+func startSleeperIn(t *testing.T, dir string) *exec.Cmd {
+	t.Helper()
+	ping := filepath.Join(os.Getenv("SystemRoot"), "System32", "ping.exe")
+	cmd := exec.Command(ping, "-n", "60", "127.0.0.1")
+	cmd.Dir = dir
+	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
+	require.NoError(t, cmd.Start())
+	t.Cleanup(func() {
+		_ = cmd.Process.Kill()
+		_, _ = cmd.Process.Wait()
+	})
+	return cmd
+}
+
+// processCWD opens pid and reads its working directory. Test-only; the reaper
+// shares one handle across read and kill via processCWDHandle.
+func processCWD(pid uint32) (string, error) {
+	h, err := windows.OpenProcess(windows.PROCESS_QUERY_INFORMATION|windows.PROCESS_VM_READ, false, pid)
+	if err != nil {
+		return "", err
+	}
+	defer func() { _ = windows.CloseHandle(h) }()
+	return processCWDHandle(h)
+}
+
+// awaitCWD waits until pid's PEB reports a readable cwd (not populated the
+// instant the process starts) and returns it.
+func awaitCWD(t *testing.T, pid int) string {
+	t.Helper()
+	var cwd string
+	require.Eventually(t, func() bool {
+		var err error
+		cwd, err = processCWD(uint32(pid))
+		return err == nil && cwd != ""
+	}, 5*time.Second, 50*time.Millisecond, "process cwd should become readable")
+	return cwd
+}
+
+func TestProcessCWDReadsWorkingDirectory(t *testing.T) {
+	dir := t.TempDir()
+	cmd := startSleeperIn(t, dir)
+	cwd := awaitCWD(t, cmd.Process.Pid)
+	require.True(t, dirContainsPath(dir, cwd), "cwd %q should be within %q", cwd, dir)
+}
+
+func TestKillProcessesWithCWDUnderTerminatesMatch(t *testing.T) {
+	dir := t.TempDir()
+	// Executable outside dir and arguments carry no workspace path, so only the
+	// working directory links this process to dir.
+	cmd := startSleeperIn(t, dir)
+	awaitCWD(t, cmd.Process.Pid)
+
+	killed, err := KillProcessesWithCWDUnder(context.Background(), []string{dir})
+	require.NoError(t, err)
+	require.GreaterOrEqual(t, killed, 1)
+
+	require.Eventually(t, func() bool {
+		return !processAlive(cmd.Process.Pid)
+	}, 5*time.Second, 50*time.Millisecond, "matched process should exit")
+}
+
+func TestKillProcessesWithCWDUnderSparesUnrelated(t *testing.T) {
+	workspace := t.TempDir()
+	elsewhere := t.TempDir()
+	cmd := startSleeperIn(t, elsewhere)
+	awaitCWD(t, cmd.Process.Pid)
+
+	_, err := KillProcessesWithCWDUnder(context.Background(), []string{workspace})
+	require.NoError(t, err)
+	require.True(t, processAlive(cmd.Process.Pid), "process outside owned dirs must be spared")
+}