Make the type of participant$ stricter

It is, in fact, required to be a behavior.

src/state/MediaViewModel.ts

@@ -259,7 +259,7 @@ abstract class BaseMediaViewModel {
     public readonly userId: string,
     // We don't necessarily have a participant if a user connects via MatrixRTC but not (yet) through
     // livekit.
-    protected readonly participant$: Observable<
+    protected readonly participant$: Behavior<
       LocalParticipant | RemoteParticipant | null
     >,
 
@@ -410,7 +410,7 @@ abstract class BaseUserMediaViewModel extends BaseMediaViewModel {
      * The expected identity of the LiveKit participant. Exposed for debugging.
      */
     public readonly rtcBackendIdentity: string,
-    participant$: Observable<LocalParticipant | RemoteParticipant | null>,
+    participant$: Behavior<LocalParticipant | RemoteParticipant | null>,
     encryptionSystem: EncryptionSystem,
     livekitRoom$: Behavior<LivekitRoom | undefined>,
     focusUrl$: Behavior<string | undefined>,
@@ -678,7 +678,7 @@ export class RemoteUserMediaViewModel extends BaseUserMediaViewModel {
     id: string,
     userId: string,
     rtcBackendIdentity: string,
-    participant$: Observable<RemoteParticipant | null>,
+    participant$: Behavior<RemoteParticipant | null>,
     encryptionSystem: EncryptionSystem,
     livekitRoom$: Behavior<LivekitRoom | undefined>,
     focusUrl$: Behavior<string | undefined>,
@@ -780,7 +780,7 @@ export class ScreenShareViewModel extends BaseMediaViewModel {
     scope: ObservableScope,
     id: string,
     userId: string,
-    participant$: Observable<LocalParticipant | RemoteParticipant>,
+    participant$: Behavior<LocalParticipant | RemoteParticipant>,
     encryptionSystem: EncryptionSystem,
     livekitRoom$: Behavior<LivekitRoom | undefined>,
     focusUrl$: Behavior<string | undefined>,