perf: remove useless stat checking logic

[HynoR]

What this PR does / why we need it?

每一个请求都stat一遍有点太浪费了，而且没有必要。

Summary of your change

移除os.stat，换成超时 dial 逻辑，超时即报 502 错误。测试 agent 挂掉时这个新逻辑不影响 core，agent再启动也没有影响

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

[f2c-ci-robot]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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