feat(FN-5189): complete Step 3 — migrate supervised spawns

Fusion-Task-Id: FN-5189
Fusion-Task-Lineage: 4caa3f0a-af81-4c60-88e8-de229ed72e08

packages/core/src/process-supervisor.ts

@@ -14,6 +14,8 @@ type ShutdownReason =
   | { kind: "test"; label: string };
 
 export interface SuperviseSpawnOptions extends Omit<SpawnOptions, "detached"> {
+  /** Override spawn for tests or alternate process factories. */
+  spawnImpl?: typeof spawn;
   /**
    * Grace period between SIGTERM and SIGKILL when the supervisor tears a child
    * down because the parent is exiting or a lifetime limit expires.
@@ -246,11 +248,12 @@ export function superviseSpawn(
   const {
     killGraceMs = DEFAULT_KILL_GRACE_MS,
     maxLifetimeMs = DEFAULT_MAX_LIFETIME_MS,
+    spawnImpl = spawn,
     ...spawnOptions
   } = options;
 
   const processGroup = usesProcessGroup();
-  const child = spawn(command, [...args], {
+  const child = spawnImpl(command, [...args], {
     ...spawnOptions,
     detached: processGroup,
   });

packages/core/src/run-command.ts

@@ -1,4 +1,4 @@
-import { spawn } from "node:child_process";
+import { superviseSpawn } from "./process-supervisor.js";
 
 export interface RunCommandOptions {
   cwd?: string;
@@ -48,27 +48,18 @@ export function runCommandAsync(
     let bufferExceeded = false;
     let timedOut = false;
     let forceKillTimer: NodeJS.Timeout | null = null;
-    const useProcessGroup = process.platform !== "win32";
 
-    const child = spawn(command, {
+    const supervised = superviseSpawn(command, [], {
       cwd: options.cwd,
       env: options.env,
-      detached: useProcessGroup,
       shell: true,
       stdio: ["ignore", "pipe", "pipe"],
+      maxLifetimeMs: (options.timeoutMs ?? 0) > 0 ? options.timeoutMs! + FORCE_KILL_DELAY_MS + 1_000 : undefined,
     });
+    const child = supervised.child;
 
     const signalProcessGroup = (signal: NodeJS.Signals): void => {
-      if (!child.pid) return;
-      try {
-        if (useProcessGroup) {
-          process.kill(-child.pid, signal);
-        } else {
-          child.kill(signal);
-        }
-      } catch {
-        // The command may already have exited and cleaned up its process group.
-      }
+      supervised.kill(signal);
     };
 
     const scheduleForceKill = (delayMs: number): void => {