[Bug]: account for additional overhead such as the ​memory cache​ and the ​vector pool when controlling memory for merge executor

[XuPeng-SH]

Is there an existing issue for the same bug?

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Branch Name

main,3.0-dev

Commit ID

latest

Other Environment Information

- Hardware parameters:
- OS type:
- Others:

Actual Behavior

No control

Expected Behavior

No response

Steps to Reproduce

25G Memory Limit and 5G Memory Cache
Run TPCH 1T

Additional information

No response