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/media-backend/livekit/RemotePublishTransport.ts

@@ -0,0 +1,90 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { type Logger } from "matrix-js-sdk/lib/logger";
+import {
+  type CallMembership,
+  getRemoteLivekitPublishCandidates,
+  type LivekitPublishCandidate,
+  type Transport,
+} from "matrix-js-sdk/lib/matrixrtc";
+import { type CallMembershipIdentityParts } from "matrix-js-sdk/lib/matrixrtc/EncryptionManager";
+
+import { isCsApiUnavailable } from "./auth";
+import * as csApi from "./auth/csApi";
+import { decodeSFUJWTPayload } from "./auth/types";
+
+export interface RemotePublishTransportParams {
+  client: csApi.ClientGetTokenParts;
+  /** The members whose SFUs are the candidates. */
+  memberships: CallMembership[];
+  ownMembershipIdentity: CallMembershipIdentityParts;
+  roomId: string;
+  slotId: string;
+  logger: Logger;
+}
+
+/**
+ * The SFU of another member that its homeserver lets us publish on, as the
+ * transport to advertise, or null where no member's SFU will do. Candidates
+ * are tried oldest membership first, and each one is checked by asking for a
+ * publisher token through our homeserver (MSC4195): a homeserver may restrict
+ * remote publishers, which shows as a token without the publish grant.
+ */
+export async function findRemotePublishTransport({
+  client,
+  memberships,
+  ownMembershipIdentity,
+  roomId,
+  slotId,
+  logger,
+}: RemotePublishTransportParams): Promise<Transport | null> {
+  logger.info(
+    "No local transport available, looking for another member's SFU to publish on",
+  );
+  const candidates = getRemoteLivekitPublishCandidates(
+    memberships,
+    ownMembershipIdentity,
+  );
+  for (const candidate of candidates) {
+    const outcome = await tryCandidate(candidate);
+    if (outcome === "usable") return candidate.transport;
+    if (outcome === "noCsApi") return null;
+  }
+  return null;
+
+  /** "noCsApi" ends the search: without the endpoint no candidate can work. */
+  async function tryCandidate({
+    url,
+    serverName,
+  }: LivekitPublishCandidate): Promise<"usable" | "unusable" | "noCsApi"> {
+    try {
+      const { jwt } = await csApi.getSFUConfig({
+        client,
+        membership: ownMembershipIdentity,
+        url,
+        serverName,
+        roomId,
+        slotId,
+      });
+      if (decodeSFUJWTPayload(jwt).video.canPublish) {
+        logger.info(`Publishing on remote SFU ${url} of ${serverName}`);
+        return "usable";
+      }
+      logger.info(`${serverName} does not let us publish on ${url}, skipping`);
+    } catch (e) {
+      if (isCsApiUnavailable(e)) {
+        logger.warn(
+          `Cannot use the MSC4195 get_token endpoint (${e}), so no remote SFU can be used`,
+        );
+        return "noCsApi";
+      }
+      logger.warn(`Failed to get a token for ${url} of ${serverName}:`, e);
+    }
+    return "unusable";
+  }
+}