fix: split Queue fields and clone sender in threads3.rs (fix #2268)

[KaustubhOG]

Closes #2268

Fixes ownership issue in send_tx where the Queue struct was moved
into the first thread closure, preventing the second thread from accessing it.

Changes:

• Destructured Queue into first_half and second_half
• Cloned mpsc::Sender so both threads can send messages concurrently

All tests pass locally:

rustc --test exercises/20_threads/threads3.rs && ./threads3

=> test result: ok. 1 passed; 0 failed

[KaustubhOG]

Hi!

All checks have passed, there are no merge conflicts, and the PR fully addresses the linked issue.
Whenever you have a moment, @rust-lang/rustlings-maintainers, could you please review this?

Happy to make any updates or adjustments if needed. Thanks for your time and work on Rustlings! 🙌

[mo8it]

See #2268 (comment)