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.ts

@@ -8,6 +8,8 @@ import {
   type IWidgetApiErrorResponseDataDetails,
   type ISearchUserDirectoryResult,
   type IGetMediaConfigResult,
+  type IRtcTransportsResult,
+  type IRtcTransport,
   OpenIDRequestState,
   SimpleObservable,
   IOpenIDUpdate,
@@ -79,29 +81,18 @@ export class CallWidgetDriver extends WidgetDriver {
     return { roomId, eventId: r.event_id };
   }
 
+  // matrix-widget-api >= 1.18 dropped the `parentDelayId` argument (MSC4140
+  // parent delays were removed from the spec draft); the signature is now
+  // (delay, eventType, content, stateKey?, roomId?).
   public async sendDelayedEvent(
-    delay: number | null,
-    parentDelayId: string | null,
+    delay: number,
     eventType: string,
-    content: IContent,
+    content: unknown,
     stateKey: string | null = null,
     targetRoomId: string | null = null,
   ): Promise<ISendDelayedEventDetails> {
     const roomId = targetRoomId || this.inRoomId;
-
-    let delayOpts;
-    if (delay !== null) {
-      delayOpts = {
-        delay,
-        ...(parentDelayId !== null && { parent_delay_id: parentDelayId }),
-      };
-    } else if (parentDelayId !== null) {
-      delayOpts = {
-        parent_delay_id: parentDelayId,
-      };
-    } else {
-      throw new Error('Must provide at least one of delay or parentDelayId');
-    }
+    const delayOpts = { delay };
 
     let r: SendDelayedEventResponse | null;
     if (stateKey !== null) {
@@ -295,6 +286,36 @@ export class CallWidgetDriver extends WidgetDriver {
     };
   }
 
+  /**
+   * MSC4515: the embedded Element Call asks us for the RTC transports because
+   * a widget can't make authenticated homeserver calls itself. Prefer the
+   * homeserver's MSC4143 `/rtc/transports` endpoint; fall back to the
+   * `org.matrix.msc4143.rtc_foci` list in the client .well-known (what
+   * Synapse deployments without MSC4143 advertise today, incl. ours).
+   * Returning an empty list makes EC report "Call is not supported".
+   */
+  public async getRtcTransports(): Promise<IRtcTransportsResult> {
+    try {
+      const fromServer = await this.mx._unstable_getRTCTransports();
+      if (Array.isArray(fromServer) && fromServer.length > 0) {
+        return { rtc_transports: fromServer as unknown as IRtcTransport[] };
+      }
+    } catch {
+      // 404 / M_UNRECOGNIZED on homeservers without MSC4143 — fall through.
+    }
+    const wellKnown = this.mx.getClientWellKnown() as
+      | { 'org.matrix.msc4143.rtc_foci'?: unknown }
+      | undefined;
+    const foci = wellKnown?.['org.matrix.msc4143.rtc_foci'];
+    const transports = Array.isArray(foci)
+      ? (foci.filter(
+          (f): f is IRtcTransport =>
+            !!f && typeof f === 'object' && typeof (f as { type?: unknown }).type === 'string',
+        ) as IRtcTransport[])
+      : [];
+    return { rtc_transports: transports };
+  }
+
   public async getMediaConfig(): Promise<IGetMediaConfigResult> {
     return this.mx.getMediaConfig();
   }