refactor: remove redundant _instant_backoff test helper

exp_backoff is already a pure generator with no sleep, so patching it
with an identical helper added no value. asyncio.sleep is mocked separately.

Author: aliev

tests/test_connection_manager.py

@@ -6,12 +6,6 @@
 from getstream.video.rtc.pb.stream.video.sfu.models import models_pb2
 
 
-async def _instant_backoff(max_retries, base=1.0, factor=2.0):
-    """exp_backoff replacement that yields without delay."""
-    for attempt in range(max_retries):
-        yield base * (factor**attempt)
-
-
 @pytest.fixture
 def connection_manager(request):
     """Create a ConnectionManager with mocked heavy dependencies.
@@ -27,7 +21,6 @@ def connection_manager(request):
         patch("getstream.video.rtc.connection_manager.SubscriptionManager"),
         patch("getstream.video.rtc.connection_manager.ParticipantsState"),
         patch("getstream.video.rtc.connection_manager.Tracer"),
-        patch("getstream.video.rtc.connection_manager.exp_backoff", _instant_backoff),
         patch(
             "getstream.video.rtc.connection_manager.asyncio.sleep",
             new_callable=AsyncMock,