test(player): hold the session stop and the kept-point rule to the server

- The client's resume thresholds equal the server's (read from both sources).
- The session stop's URL matches the route the server serves it on.
- The player stops its server session by its item and its own id on an unload and on close,
  and keeps it when the page only goes into the back/forward cache.

frontend/src/lib/api/plexStream.contract.test.ts

@@ -0,0 +1,36 @@
+// @vitest-environment jsdom
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+import { afterEach, expect, it, vi } from "vitest";
+import { api } from "../api";
+
+// The session stop is a keepalive beacon: nothing answers it on screen, so a URL that no longer
+// matches the server's route fails silently — every transcode the player leaves runs on until the
+// idle reaper. The route is read from the server's source, as the other contract tests do.
+afterEach(() => vi.restoreAllMocks());
+
+function serverRoute(): RegExp {
+  // From the frontend root (vitest's cwd): this file runs under jsdom — `api` boots i18n, which reads
+  // localStorage — and there `import.meta.url` is not a file URL `readFileSync` takes.
+  const source = readFileSync(
+    join(process.cwd(), "../backend/app/routes/plex/playback.py"),
+    "utf8",
+  );
+  const prefix = /APIRouter\(prefix="([^"]+)"/.exec(source)?.[1];
+  const path = /@router\.post\("(\/stream\/[^"]*\/stop)"\)/.exec(source)?.[1];
+  // If either is missing the route was renamed or moved — re-point this test, the rule stands.
+  expect(prefix, "router prefix").toBeDefined();
+  expect(path, "the session stop route").toBeDefined();
+  const pattern = (prefix! + path!).replace(/\{[^}]+\}/g, "[^/]+");
+  return new RegExp(`^${pattern}$`);
+}
+
+it("sends the session stop to the route the server serves it on", () => {
+  const fetch = vi.spyOn(globalThis, "fetch").mockResolvedValue(new Response(null));
+  api.plexStopSession("12345", "a1b2c3");
+  const url = String(fetch.mock.calls[0]![0]);
+  expect(url).toMatch(serverRoute());
+  expect(url).toContain("/12345/");
+  expect(url).toContain("/a1b2c3/");
+  expect(fetch.mock.calls[0]![1]).toMatchObject({ method: "POST", keepalive: true });
+});