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/components/PlexPlayer.session.dom.test.tsx

@@ -731,3 +731,38 @@ it("a session answer that lands while the next item's detail is still loading is
   await tick();
   expect(video().getAttribute("src")).not.toBe("/media/B");
 });
+
+describe("the server session the player no longer wants", () => {
+  const hlsA = { mode: "hls", url: "/hls/A/index.m3u8", start_s: 0, id: "sid-A" } as Awaited<
+    ReturnType<typeof api.plexSession>
+  >;
+
+  it("is stopped by its item and its own id when the page is unloaded", async () => {
+    const stop = vi.spyOn(api, "plexStopSession").mockImplementation(() => {});
+    mount({ details: { A: item("A") }, sessions: { A: () => Promise.resolve(hlsA) } });
+    await waitFor(() => expect(video()?.getAttribute("src")).toBe("/hls/A/index.m3u8"));
+    act(
+      () => void window.dispatchEvent(Object.assign(new Event("pagehide"), { persisted: false })),
+    );
+    expect(stop).toHaveBeenCalledWith("A", "sid-A");
+  });
+
+  it("is stopped the same way when the player closes", async () => {
+    const stop = vi.spyOn(api, "plexStopSession").mockImplementation(() => {});
+    const { unmount } = mount({
+      details: { A: item("A") },
+      sessions: { A: () => Promise.resolve(hlsA) },
+    });
+    await waitFor(() => expect(video()?.getAttribute("src")).toBe("/hls/A/index.m3u8"));
+    unmount();
+    expect(stop).toHaveBeenCalledWith("A", "sid-A");
+  });
+
+  it("is kept when the page only goes into the back/forward cache", async () => {
+    const stop = vi.spyOn(api, "plexStopSession").mockImplementation(() => {});
+    mount({ details: { A: item("A") }, sessions: { A: () => Promise.resolve(hlsA) } });
+    await waitFor(() => expect(video()?.getAttribute("src")).toBe("/hls/A/index.m3u8"));
+    act(() => void window.dispatchEvent(Object.assign(new Event("pagehide"), { persisted: true })));
+    expect(stop).not.toHaveBeenCalled();
+  });
+});