fix: enforce one Pi runtime per user

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

@@ -95,6 +95,47 @@ test("oltre maxPiProcesses solleva errore", async () => {
   mgr.teardown("a");
 });
 
+test("teardownForPrincipal stops only runtimes owned by that user", () => {
+  const aliceChild = recordingChild();
+  const bobChild = recordingChild();
+  aliceChild.kill = vi.fn();
+  bobChild.kill = vi.fn();
+  const children = [aliceChild, bobChild];
+  const mgr = new PiProcessManager(loadConfig({}), { spawnFn: () => children.shift() as any });
+  const alice = { issuer: "portal", subject: "alice", isAdmin: false };
+  const bob = { issuer: "portal", subject: "bob", isAdmin: false };
+  mgr.createFor("alice-session", { principal: alice });
+  mgr.createFor("bob-session", { principal: bob });
+
+  expect(mgr.teardownForPrincipal(alice)).toEqual(["alice-session"]);
+
+  expect(mgr.get("alice-session")).toBeUndefined();
+  expect(mgr.get("bob-session")).toBeDefined();
+  expect(aliceChild.kill).toHaveBeenCalledOnce();
+  expect(bobChild.kill).not.toHaveBeenCalled();
+  mgr.teardown("bob-session");
+});
+
+test("createFor keeps at most one runtime for the same user", () => {
+  const firstChild = recordingChild();
+  const secondChild = recordingChild();
+  firstChild.kill = vi.fn();
+  secondChild.kill = vi.fn();
+  const children = [firstChild, secondChild];
+  const mgr = new PiProcessManager(loadConfig({}), { spawnFn: () => children.shift() as any });
+  const principal = { issuer: "portal", subject: "alice", isAdmin: false };
+
+  mgr.createFor("first", { principal });
+  const second = mgr.createFor("second", { principal });
+
+  expect(mgr.count()).toBe(1);
+  expect(mgr.get("first")).toBeUndefined();
+  expect(mgr.get("second")).toBe(second);
+  expect(firstChild.kill).toHaveBeenCalledOnce();
+  expect(secondChild.kill).not.toHaveBeenCalled();
+  mgr.teardown("second");
+});
+
 function recordingChild() {
   const ch: any = new EventEmitter();
   ch.stdout = new EventEmitter();