fix(s04): use early-continue idiom in problem code block

The problem block used a nested `if block.type == "tool_use":` while the
solution block and code.py use `if block.type != "tool_use": continue`.
This made the problem->solution diff show two changes instead of the one
the section teaches (the README states only one place in the loop changed).
Align all three README variants (zh/en/ja) to the early-continue idiom.