fix(call): serve RTC transports to Element Call over MSC4515 — calls work again

Since the upstream v0.25.0 merge, Element Call in widget mode no longer
reads .well-known for its LiveKit transport; it asks the HOST via the
widget API (org.matrix.msc4515.get_rtc_transports, capability
org.matrix.msc4515.rtc_transports). cinny never granted the capability
nor implemented WidgetDriver.getRtcTransports(), so discovery returned
nothing and every join failed with "Call is not supported"
(MISSING_MATRIX_RTC_TRANSPORT) — the [LocalMembership] Multiple
Transport Errors line in the browser console.

- matrix-widget-api 1.17.0 -> 1.18.0 (adds MSC4515; also changes the
  sendDelayedEvent driver signature, adapted — parent delay ids were
  removed from the draft).
- Grant MSC4515RtcTransports in getCallCapabilities.
- CallWidgetDriver.getRtcTransports(): homeserver /rtc/transports
  (MSC4143) first, then the .well-known org.matrix.msc4143.rtc_foci list
  (what matrix.lotusguild.org advertises today). Unit-tested.

Server side needs no change: the well-known already carries the livekit
focus and the JWT service answers.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/plugins/call/CallWidgetDriver.rtcTransports.test.ts

@@ -0,0 +1,58 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import type { MatrixClient } from 'matrix-js-sdk';
+import { CallWidgetDriver } from './CallWidgetDriver';
+
+// MSC4515: Element Call (widget mode) asks the host for RTC transports. Without a
+// working answer every call fails with MISSING_MATRIX_RTC_TRANSPORT.
+function fakeClient(opts: {
+  serverTransports?: unknown[] | Error;
+  wellKnown?: Record<string, unknown>;
+}): MatrixClient {
+  return {
+    getDeviceId: () => 'DEV',
+    getSafeUserId: () => '@me:example.org',
+    _unstable_getRTCTransports: async () => {
+      if (opts.serverTransports instanceof Error) throw opts.serverTransports;
+      return opts.serverTransports ?? [];
+    },
+    getClientWellKnown: () => opts.wellKnown,
+  } as unknown as MatrixClient;
+}
+
+const focus = { type: 'livekit', livekit_service_url: 'https://matrix.example.org' };
+
+test('getRtcTransports prefers the homeserver MSC4143 endpoint', async () => {
+  const driver = new CallWidgetDriver(fakeClient({ serverTransports: [focus] }), '!r:x');
+  const r = await driver.getRtcTransports();
+  assert.deepEqual(r.rtc_transports, [focus]);
+});
+
+test('getRtcTransports falls back to .well-known rtc_foci when the endpoint 404s', async () => {
+  const driver = new CallWidgetDriver(
+    fakeClient({
+      serverTransports: new Error('404'),
+      wellKnown: { 'org.matrix.msc4143.rtc_foci': [focus, { junk: true }, null] },
+    }),
+    '!r:x',
+  );
+  const r = await driver.getRtcTransports();
+  assert.deepEqual(r.rtc_transports, [focus]);
+});
+
+test('getRtcTransports returns an empty list when nothing is advertised', async () => {
+  const driver = new CallWidgetDriver(
+    fakeClient({ serverTransports: [], wellKnown: { 'm.homeserver': {} } }),
+    '!r:x',
+  );
+  const r = await driver.getRtcTransports();
+  assert.deepEqual(r.rtc_transports, []);
+});
+
+test('the MSC4515 capability is granted to the call widget', async () => {
+  const driver = new CallWidgetDriver(fakeClient({}), '!r:x');
+  const allowed = await driver.validateCapabilities(
+    new Set(['org.matrix.msc4515.rtc_transports', 'org.example.not_allowed']),
+  );
+  assert.deepEqual(Array.from(allowed), ['org.matrix.msc4515.rtc_transports']);
+});