Fix session_manager_ref bug and add manager unit tests

Fix a bug where `session_manager_ref or []` would create a new empty list
when an empty list was passed (since empty list is falsy), breaking the
session manager reference chain. This prevented close_sse_stream from
being called during SSE polling tests.

Also add unit tests for StreamableHTTPSessionManager.close_sse_stream to
ensure proper coverage of lines 297-301 in streamable_http_manager.py.

Author: felixweinberger

tests/shared/test_streamable_http.py

@@ -118,7 +118,7 @@ class ServerTest(Server):  # pragma: no cover
     def __init__(self, session_manager_ref: list[StreamableHTTPSessionManager] | None = None):
         super().__init__(SERVER_NAME)
         self._lock = None  # Will be initialized in async context
-        self._session_manager_ref = session_manager_ref or []
+        self._session_manager_ref = [] if session_manager_ref is None else session_manager_ref
 
         @self.read_resource()
         async def handle_read_resource(uri: AnyUrl) -> str | bytes:
@@ -1798,6 +1798,42 @@ async def test_close_sse_stream_already_closed():
     assert "request-456" not in transport._request_streams
 
 
+@pytest.mark.anyio
+async def test_manager_close_sse_stream():
+    """Test StreamableHTTPSessionManager.close_sse_stream delegates to transport."""
+    from unittest.mock import AsyncMock
+
+    # Create a minimal server for the manager
+    server = Server("test-manager")
+    manager = StreamableHTTPSessionManager(app=server)
+
+    # Create a mock transport and add it to _server_instances
+    mock_transport = AsyncMock()
+    mock_transport.close_sse_stream = AsyncMock()
+    manager._server_instances["test-session-123"] = mock_transport
+
+    # Call close_sse_stream on the manager
+    result = await manager.close_sse_stream("test-session-123", "request-456")
+
+    # Should return True and delegate to transport
+    assert result is True
+    mock_transport.close_sse_stream.assert_awaited_once_with("request-456")
+
+
+@pytest.mark.anyio
+async def test_manager_close_sse_stream_session_not_found():
+    """Test StreamableHTTPSessionManager.close_sse_stream returns False for unknown session."""
+    # Create a minimal server for the manager
+    server = Server("test-manager")
+    manager = StreamableHTTPSessionManager(app=server)
+
+    # No sessions in _server_instances
+    result = await manager.close_sse_stream("nonexistent-session", "request-123")
+
+    # Should return False
+    assert result is False
+
+
 @pytest.mark.anyio
 async def test_resume_stream_without_session_id():
     """Test resume_stream returns early without session ID."""