Fix: close MCP sessions after canvas execution to prevent connection leaks

Add Graph.close() method that finds and closes all MCPToolCallSessions
held by Agent components. Call it in finally blocks after canvas.run()
in canvas_service.py and canvas_app.py. Also move session cleanup to
finally blocks in test_tool endpoint and get_mcp_tools to ensure
sessions are closed even on exceptions.

agent/canvas.py

@@ -137,6 +137,20 @@ class Graph:
         except Exception as e:
             logging.exception(e)
 
+    def close(self):
+        from common.mcp_tool_call_conn import MCPToolCallSession
+        seen = set()
+        for cpn in self.components.values():
+            obj = cpn.get("obj")
+            if obj and hasattr(obj, "tools"):
+                for tool in obj.tools.values():
+                    if isinstance(tool, MCPToolCallSession) and id(tool) not in seen:
+                        seen.add(id(tool))
+                        try:
+                            tool.close_sync(timeout=3)
+                        except Exception:
+                            pass
+
     def get_component_name(self, cid):
         for n in self.dsl.get("graph", {}).get("nodes", []):
             if cid == n["id"]: