handle connection close on CancelledError to prevent stale results

[shenchucheng]

Description

This PR resolves an issue where canceled queries using connections from a pool leave unintended results that affect subsequent queries. When a query is canceled, the connection is returned to the pool without proper cleanup, causing future queries to receive residual data from the canceled task.

Solution

• Implemented _close_on_cancel to ensure that connections are fully closed and reset when a query is canceled.
• Added a test case to verify that canceled queries do not impact future connections from the pool.

This update ensures clean connection states on cancellation, improving reliability in pooled query operations. This approach is inspired by similar handling in aiomysql

Related Issues

• Fixes 当请求被取消的时候，任务被CancelledError，但是连接没有被正确回收 #100
• Fixes The user actively cancels the request, the task is canceled by CancelledError(), and the connection object or cursor recovery fails, causing the new SQL query to get the SQL result of the previous request. #103

[long2ice]

Please run make style ci in local first

[shenchucheng]

Please run make style ci in local first

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