Use compare_exchange_weak in steal loop for potential performance imp…

[xin-ok]

What problem does this PR solve?

Issue Number:

Problem Summary:
在 WorkStealingQueue 的 steal 函数中，使用 compare_exchange_strong 在循环中重试。
由于已经在 do-while 循环中，改用 compare_exchange_weak 可以获得更好的性能，
同时保持语义正确

What is changed and the side effects?

Changed:

• 将 _top.compare_exchange_strong 改为 _top.compare_exchange_weak
• 保持其他逻辑不变

Side effects:

• Performance effects:
• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
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[Copilot]

Pull request overview

This PR makes a small performance-oriented tweak to bthread::WorkStealingQueue::steal() by switching the CAS operation in the retry loop from compare_exchange_strong to compare_exchange_weak, which is safe in a loop and may reduce overhead on some architectures while preserving correctness.

Changes:

• Replace _top.compare_exchange_strong(...) with _top.compare_exchange_weak(...) inside the steal() do/while retry loop.
• Keep memory orders and the rest of the steal logic unchanged.

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