Fall back to publishing on a remote SFU if possible

Port of edf1feae from johannes/rtc-remote-publish onto the SDK split,
where
the MatrixRTC logic lives in sdk/ rather than src/state/CallViewModel/.

Where the original lands:

- getLocalTransport's remote fallback: the LiveKit backend's new
  findRemotePublishTransport (sdk/src/media-backend/livekit/
  RemotePublishTransport.ts), which tries the candidates from the
  js-sdk's
  getRemoteLivekitPublishCandidates oldest first and keeps the first
  whose
  homeserver issues a token with the publish grant.
  discoverLocalTransport
  asks the backend for it after the host's fallback, with a snapshot of
  the
  session's memberships from RTCParticipation, and throws
  MatrixRTCTransportMissingError where that finds nothing too. Transport
  discovery stays backend neutral: the MSC4195 token check is the
  backend's.
- LocalMember's "no delegation on a remote SFU" and the custom
  transport's
  server_name: LivekitBackend.prepareLocalTransport locates the
  transport on
  the homeserver it names (serverNameOfTransport) and skips the
  delegation
  probe where that is not our own.
- SessionBehaviors' membership.getTransportServerName: the same helper
  in
  LivekitBackend.mediaFor.
- decodeSFUJWTPayload, SFUJWTPayload.video.canPublish and the exported
  isCsApiUnavailable: sdk/src/media-backend/livekit/auth/.
- DeveloperSettingsTab, the widget capabilities, the locale string and
  the
  js-sdk pin (9405287c, 44.0.0): applied as is. The error boundary
  snapshots follow the string.

Tests: the LocalTransport cases go to RemotePublishTransport.test.ts and
LocalTransport.test.ts, the LocalMember remote case and the
SessionBehaviors
server-name case to LivekitBackend.test.ts, plus the memberships
snapshot in
RTCParticipation.test.ts.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

sdk/src/RTCParticipation.test.ts

@@ -84,6 +84,7 @@ interface FakeBackend {
   connectionState$: BehaviorSubject<MediaConnectionState | Error>;
   data$: Subject<DataPacket>;
   setPublishing: Mock<(publish: boolean) => void>;
+  findRemotePublishTransport: Mock<() => Promise<Transport | null>>;
   delegateDelayedLeave: Mock<(delayId: string) => Promise<void>>;
   mediaFor: Mock<(scope: ObservableScope, member: RTCMember) => MemberMedia>;
 }
@@ -91,6 +92,7 @@ interface FakeBackend {
 function setup(
   options: Partial<RTCParticipationOptions> = {},
   slotOptions: { matrixRTCMode?: MatrixRTCMode } = {},
+  homeserverTransports: Transport[] = [transport],
 ): {
   scope: ObservableScope;
   participation: ReturnType<typeof createRTCParticipation>["participation"];
@@ -112,7 +114,7 @@ function setup(
     getDeviceId: () => deviceId,
     getDomain: () => "example.org",
     getSyncState: () => SyncState.Syncing,
-    _unstable_getRTCTransports: vi.fn().mockResolvedValue([transport]),
+    _unstable_getRTCTransports: vi.fn().mockResolvedValue(homeserverTransports),
   } as unknown as MatrixClient;
   const members$ = new BehaviorSubject<RTCMember[]>([]);
 
@@ -124,6 +126,7 @@ function setup(
     ),
     data$: new Subject<DataPacket>(),
     setPublishing: vi.fn(),
+    findRemotePublishTransport: vi.fn(async () => Promise.resolve(null)),
     delegateDelayedLeave: vi.fn(async () => Promise.resolve()),
     mediaFor: vi.fn(() => ({
       tracks$: constant(null),
@@ -136,6 +139,7 @@ function setup(
   ): MediaBackend => {
     const backend: MediaBackend = {
       transportType: "livekit",
+      findRemotePublishTransport: fake.findRemotePublishTransport,
       prepareLocalTransport: async () => fake.prepared.promise,
       delegateDelayedLeave: fake.delegateDelayedLeave,
       local: {
@@ -220,6 +224,27 @@ describe("createRTCParticipation", () => {
     });
   });
 
+  it("publishes on another member's SFU where the homeserver advertises none", async () => {
+    const bobsTransport: Transport = {
+      type: "livekit",
+      url: "wss://sfu.bob.com",
+      server_name: "bob.com",
+    };
+    const { session, join, fake } = setup({}, {}, []);
+    fake.findRemotePublishTransport.mockResolvedValue(bobsTransport);
+    fake.prepared.resolve({ canDelegateDelayedLeave: false });
+    await flushPromises();
+
+    expect(fake.findRemotePublishTransport).toHaveBeenCalledWith(
+      session.memberships,
+    );
+    expect(join).toHaveBeenCalledWith(
+      expect.anything(),
+      [bobsTransport],
+      expect.anything(),
+    );
+  });
+
   it("publishes only while joined", async () => {
     const { participation, session, fake, scope } = setup();
     const ended = vi.fn();