fix: release finalized Pi runtimes

backend/test/routes-sessions.test.ts

@@ -1304,6 +1304,47 @@ test("an unexpectedly exited runtime publishes its terminal sequence then releas
   ]);
 });
 
+test("agent_end releases the Pi runtime after the session was finalized", async () => {
+  let listener: ((event: any) => void) | undefined;
+  const bridge = {
+    onClientEvent: (next: (event: any) => void) => { listener = next; },
+    emitClientEvent: (event: any) => listener?.(event),
+  };
+  const runtime = { bridge };
+  let current: any;
+  let teardownCalls = 0;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    mgr: {
+      get: () => current,
+      createFor: () => { current = runtime; return runtime; },
+      configure: async () => {},
+      start: () => {},
+      teardownIfCurrent: (_id: string, expected: any) => {
+        if (current !== expected) return false;
+        teardownCalls += 1;
+        current = undefined;
+        return true;
+      },
+    } as any,
+    thtRunner: {
+      sessionNew: async () => ({ id: "s1" }),
+      sessionShow: async () => ({ status: "finalized" }),
+      searchPack: async () => {},
+      failSession: async () => {},
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({ workspace: "local" }) as any,
+  });
+  await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
+  await new Promise((resolve) => setImmediate(resolve));
+
+  bridge.emitClientEvent({ type: "system_event", event: "agent_end" });
+  await new Promise((resolve) => setImmediate(resolve));
+
+  expect(teardownCalls).toBe(1);
+  expect(current).toBeUndefined();
+});
+
 test("POST resume tears down a created runtime when bridge binding fails", async () => {
   const delivered: string[] = [];
   const actualHub = new SseHub();