test(player): pin the stop URL's ids, the beacon's settle, the kept point's edges

The stop-URL contract passed with the two ids swapped and read the backend
from the working directory; nothing called a real beacon's onSettled or its
library pass-through; the kept-point rule was held by its constants only.

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

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 import { readFileSync } from "node:fs";
-import { join } from "node:path";
+import { resolve } from "node:path";
 import { afterEach, expect, it, vi } from "vitest";
 import { api } from "../api";
 
@@ -10,10 +10,9 @@ import { api } from "../api";
 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.
+  // Anchored to this file, not the working directory, so the test holds wherever vitest starts.
   const source = readFileSync(
-    join(process.cwd(), "../backend/app/routes/plex/playback.py"),
+    resolve(__dirname, "../../../../backend/app/routes/plex/playback.py"),
     "utf8",
   );
   const prefix = /APIRouter\(prefix="([^"]+)"/.exec(source)?.[1];
@@ -21,7 +20,9 @@ function serverRoute(): RegExp {
   // 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, "[^/]+");
+  // Each path parameter becomes a group of its own NAME: two ids that swapped places still match a
+  // pattern of anonymous wildcards, and the server then finds no session to stop.
+  const pattern = (prefix! + path!).replace(/\{(\w+)\}/g, "(?<$1>[^/]+)");
   return new RegExp(`^${pattern}$`);
 }
 
@@ -29,8 +30,6 @@ 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(url.match(serverRoute())?.groups).toEqual({ rating_key: "12345", session_id: "a1b2c3" });
   expect(fetch.mock.calls[0]![1]).toMatchObject({ method: "POST", keepalive: true });
 });