test: drop now-dead ResourceWarning filter on trio client test

The filter was added in #870 to suppress the receive-stream leak that
this PR fixes; with disconnect() now closing the stream, the trio test
no longer emits the warning. Verified with -W error::ResourceWarning.

Author: claude

tests/test_client.py

@@ -260,9 +260,6 @@ async def mock_receive():
         anyio.run(_test)
 
 
-@pytest.mark.filterwarnings(
-    "ignore:Unclosed <MemoryObjectReceiveStream:ResourceWarning"
-)
 class TestClaudeSDKClientTrioBackend:
     """Regression test: ClaudeSDKClient must work under trio.