fix: check for running processes before Cmd+W closes terminal

[honerlaw]

Summary

• Cmd+W now checks isTerminalBusy() before closing a terminal tab, matching the existing X button behavior
• If a process is running, a confirmation dialog is shown; otherwise the tab closes immediately
• Adds requestClose to the TerminalTabsHandle imperative handle and removes the now-unused direct closeTerminal call from useCommands

Test plan

• Open a terminal, run sleep 999, press Cmd+W — confirmation dialog should appear
• Cancel the dialog — terminal stays open
• Press Cmd+W again, confirm — terminal closes
• Open a terminal with no running process, press Cmd+W — closes immediately (no dialog)
• Verify the X button on tabs still works the same way

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