[songgaoye]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

ExecutionQueue<T>::execute(const T&, ...) casts its const lvalue input to
T& and forwards it as T&&. As a result, submitting an lvalue can invoke
T's move constructor and unexpectedly modify the caller's object.

What is changed and the side effects?

Changed:

• Replace the const_cast-based forwarding path with an internal forwarding
  helper.
• Preserve copy semantics for lvalue tasks and move semantics for rvalue tasks.
• Add a regression test verifying that submitting an lvalue does not move from
  it.

Side effects:

• Lvalue task submissions now consistently copy the task, as implied by the
  const T& API.

• Rvalue task submissions retain their existing move behavior.

• Performance effects:

  No expected impact on rvalue submissions. Lvalue submissions now avoid
  unintended moves and follow the intended copy path.

• Breaking backward compatibility:

  No API or ABI changes. Code that unintentionally relied on an lvalue being
  moved from will observe the corrected behavior.

---

Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

🟢 Approval recommended

The focused forwarding fix preserves the intended submission semantics, and the regression test covers the reported defect with no identified blocking issues.

No open issues 🎉

What changed in this PR

This PR fixes execution queue submission so lvalue tasks are copied without modifying the caller’s object, while rvalue tasks retain move semantics.

Changes:

• Replace const-cast forwarding with a private helper that preserves value categories.
• Add a regression test checking that lvalue submission does not move from the original task and produces the expected result.

File	Description
test/​bthread_execution_queue_unittest.cpp	Add a copy/move-aware task and lvalue regression test.
src/​bthread/​execution_queue_inl.h	Preserve copy and move semantics through a forwarding helper.

_{Review effort: Balanced}