hardened API decoding against server fields the models did not expect

The Talk room list crashed for a new account because the server omits
lastMessage for rooms without a visible message and the model required it.
Beyond that single field, a review of the Talk models against the OpenAPI
spec of the installed version (23.0.8) found the same crash class in several
places: closed enums over open server values (inCall bitmask, rich-object
types, actor and message types), a deleted parent sent as {id, deleted},
and the autocomplete status arriving as an object.

- json_serializable `checked: true`: decode errors now name class and field
- lists (rooms, messages, participants, autocomplete) decode per entry;
  a malformed entry is skipped and reported via ClientErrorReporter
- unknownEnumValue fallbacks for all response enums of Talk and breakers
- lastMessage nullable; inCall as bitmask; deleted parent placeholder;
  autocomplete status object/string; fixed shareWithDisplayNameUnique key
- GetRoomParams no longer sends null query parameters

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HrepDTL4qDe35TuS29sLW3

test/talk/get_participants_response_test.dart

@@ -0,0 +1,23 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:marianum_mobile/api/marianumcloud/talk/get_participants/get_participants_response.dart';
+
+void main() {
+  test('reads inCall as a bitmask', () {
+    final participant = GetParticipantsResponseObject.fromJson({
+      'attendeeId': 1,
+      'actorType': 'users',
+      'actorId': 'u',
+      'displayName': 'U',
+      'participantType': 3,
+      'lastPing': 0,
+      'inCall': 7,
+      'permissions': 0,
+      'attendeePermissions': 0,
+      'sessionIds': <String>[],
+    });
+    expect(participant.isInCall, isTrue);
+    expect(participant.inCallWithAudio, isTrue);
+    expect(participant.inCallWithVideo, isTrue);
+    expect(participant.inCallViaPhone, isFalse);
+  });
+}