refactor(backend): drop dead SpawnFn alias + test Pi exit-handler cleanup

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/pi/pi-process-manager.ts

@@ -10,13 +10,14 @@ export interface SessionRuntime {
   child: ChildProcessWithoutNullStreams;
 }
 
+/** Injected test double signature: produce a child process, no args needed. */
 type SpawnFn = () => ChildProcessWithoutNullStreams;
 
 export class PiProcessManager {
   private runtimes = new Map<string, SessionRuntime>();
   private spawnFn: (sessionId: string, author: string) => ChildProcessWithoutNullStreams;
 
-  constructor(private cfg: AppConfig, opts?: { spawnFn?: () => ChildProcessWithoutNullStreams }) {
+  constructor(private cfg: AppConfig, opts?: { spawnFn?: SpawnFn }) {
     if (opts?.spawnFn) {
       this.spawnFn = () => opts.spawnFn!();
     } else {

backend/test/pi-process-manager.test.ts

@@ -19,6 +19,21 @@ test("spawnFor avvia un runtime e il bridge emette il widget F1", async () => {
   expect(mgr.count()).toBe(0);
 });
 
+test("l'exit del child rimuove il runtime dalla mappa (exit handler)", async () => {
+  const cfg = loadConfig({ THT_HARNESS_DIR: "../harness" });
+  const mgr = new PiProcessManager(cfg, { spawnFn: () => spawn("node", [FAKE, SCRIPT]) as any });
+  const rt = await mgr.spawnFor("exit-test", {});
+  expect(mgr.count()).toBe(1);
+  // Cause the child to exit on its own and await the 'exit' event (no teardown call).
+  const exited = new Promise<void>((res) => rt.child.on("exit", () => res()));
+  rt.child.kill();
+  await exited;
+  // Let the manager's registered exit handler run.
+  await new Promise((res) => setImmediate(res));
+  expect(mgr.count()).toBe(0);
+  expect(mgr.get("exit-test")).toBeUndefined();
+});
+
 test("oltre maxPiProcesses solleva errore", async () => {
   const cfg = { ...loadConfig({}), maxPiProcesses: 1 };
   const mgr = new PiProcessManager(cfg, { spawnFn: () => spawn("node", [FAKE, SCRIPT]) as any });