gtk_media_file_set_input_stream assertion failure when using GStreamer backend

Calling gtk_media_file_set_input_stream with a valid GInputStream asserts in the gtkgstmediafile implementation.

Steps to reproduce

Write a GTK4 application, using GTK_MEDIA=gstreamer, with the following calls:

  SoupSession* session = soup_session_new();
  SoupMessage* msg = soup_message_new("GET", "http://commondatastorage.googleapis.com/gtv-videos-bucket/sample/BigBuckBunny.mp4");
  GInputStream* stream = soup_session_send(session, msg, NULL, NULL);
  
  GtkMediaStream* file = gtk_media_file_new();
  gtk_media_file_set_input_stream(GTK_MEDIA_FILE(file), stream);

Current behavior

Program crashes with the following:

ERROR:../modules/media/gtkgstmediafile.c:276:gtk_gst_media_file_open: code should not be reached

The assert in question is:

      /* It's an input stream */
      g_assert_not_reached ();

When using the ffmpeg backend instead, gtk_media_file_set_input_stream works as expected.

Expected outcome

The program should not crash, and the GtkMediaFile should be usable as if it had been initialized with gtk_media_file_set_filename.

Version information

GTK version: 4.3.0
Flatpak runtime: org.gnome.Sdk 40

Additional information

Discussed here: https://discourse.gnome.org/t/gtk4-video-playback-from-inputstream/6762