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/api.ts

@@ -13,7 +13,10 @@ Please see LICENSE in the repository root for full details.
  */
 
 import { type Logger } from "matrix-js-sdk/lib/logger";
-import { type Transport } from "matrix-js-sdk/lib/matrixrtc";
+import {
+  type CallMembership,
+  type Transport,
+} from "matrix-js-sdk/lib/matrixrtc";
 import { type CallMembershipIdentityParts } from "matrix-js-sdk/lib/matrixrtc/EncryptionManager";
 import { type Observable } from "rxjs";
 
@@ -116,6 +119,17 @@ export interface MediaBackend {
   /** The membership transport type this backend serves: "livekit". */
   readonly transportType: string;
 
+  /**
+   * A transport to publish on where neither the homeserver nor the host
+   * provides one of the backend's type: for LiveKit, the SFU of another
+   * member whose homeserver lets us publish on it (MSC4195). Resolves null
+   * where no member's transport will do. Called at most once per session,
+   * with the memberships at that moment, before `prepareLocalTransport`.
+   */
+  findRemotePublishTransport(
+    memberships: CallMembership[],
+  ): Promise<Transport | null>;
+
   /**
    * Makes `transport` usable as the one the local member publishes on. The
    * session found it; the backend does whatever has to succeed before it is