Fix: close MCP sessions after canvas execution to prevent connection leaks (#13295)

### What problem does this PR solve?

Closes #12962

MCPToolCallSessions created during agent execution (in `Agent.__init__`)
are never explicitly closed. Each session starts its own event loop
thread and opens an SSE/HTTP connection to the MCP server. When the
canvas goes out of scope, these threads and connections remain alive
indefinitely, accumulating over time and causing resource exhaustion
after prolonged use.

### Solution

1. Add a `Graph.close()` method that iterates all components, finds
MCPToolCallSessions held by Agent tools, and calls `close_sync()` on
each to properly shut down the event loop, thread, and connection.
2. Call `canvas.close()` in `finally` blocks after `canvas.run()`
completes in `canvas_service.py` and `canvas_app.py`.
3. Move MCP session cleanup to `finally` blocks in `test_tool` endpoint
(`mcp_server_app.py`) and `get_mcp_tools` (`api_utils.py`) to ensure
sessions are closed even on exceptions.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

---------

Co-authored-by: conflict-resolver <conflict-resolver@local>
Co-authored-by: Zhichang Yu <yuzhichang@gmail.com>

api/utils/api_utils.py

@@ -658,11 +658,12 @@ def get_mcp_tools(mcp_servers: list, timeout: float | int = 10) -> tuple[dict, s
                 tool_dict["enabled"] = cached_tool.get("enabled", True)
                 results[server_key].append(tool_dict)
 
-        # PERF: blocking call to close sessions — consider moving to background thread or task queue
-        close_multiple_mcp_toolcall_sessions(tool_call_sessions)
         return results, ""
     except Exception as e:
         return {}, str(e)
+    finally:
+        # PERF: blocking call to close sessions — consider moving to background thread or task queue
+        close_multiple_mcp_toolcall_sessions(tool_call_sessions)
 
 
 async def is_strong_enough(chat_model, embedding_model):