fix: reap finalized runtimes before session start

backend/test/routes-sessions.test.ts

@@ -181,6 +181,64 @@ test("POST /sessions usa i settings (workspace/provider/model/thinking) e crea+a
   unlinkSync(modelKey);
 });
 
+test("POST /sessions reclaims a finalized runtime before enforcing the Pi limit", async () => {
+  const runtimes = new Map<string, any>();
+  const statuses = new Map<string, string>();
+  const tornDown: string[] = [];
+  const order: string[] = [];
+  let nextId = 0;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    mgr: {
+      get: (id: string) => runtimes.get(id),
+      createFor: (id: string) => {
+        order.push(`create:${id}`);
+        if (runtimes.size >= 1) throw new Error("max Pi processes reached");
+        const runtime = { bridge: { onClientEvent: () => {} } };
+        runtimes.set(id, runtime);
+        return runtime;
+      },
+      configure: async () => {},
+      start: () => {},
+      teardownIfCurrent: (id: string, expected: any) => {
+        if (runtimes.get(id) !== expected) return false;
+        runtimes.delete(id);
+        tornDown.push(id);
+        order.push(`teardown:${id}`);
+        return true;
+      },
+    } as any,
+    thtRunner: {
+      sessionNew: async () => {
+        const id = `s${++nextId}`;
+        order.push(`new:${id}`);
+        statuses.set(id, "open");
+        return { id };
+      },
+      sessionShow: async (id: string) => {
+        order.push(`show:${id}`);
+        return { status: statuses.get(id) };
+      },
+      searchPack: async () => {},
+      failSession: async () => {},
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({ workspace: "local" }) as any,
+  });
+
+  expect((await app.inject({ method: "POST", url: "/sessions", payload: { question: "one" } })).statusCode)
+    .toBe(200);
+  statuses.set("s1", "finalized");
+  order.length = 0;
+
+  const second = await app.inject({ method: "POST", url: "/sessions", payload: { question: "two" } });
+
+  expect(second.statusCode).toBe(200);
+  expect(second.json()).toEqual({ id: "s2" });
+  expect(tornDown).toEqual(["s1"]);
+  expect(order).toEqual(["show:s1", "teardown:s1", "new:s2", "create:s2"]);
+  expect(runtimes.has("s2")).toBe(true);
+});
+
 test("POST /sessions refuses to create a session when the local DWH precheck fails", async () => {
   let sessionNewCalls = 0;
   let pinged = 0;