chore: few more improvements for the academy exercises

[honzajavorek]

• removes leftover file spotted by @TC-MO
• removes workaround as The enqueue_links function misinterprets the limit keyword argument crawlee-python#1673 has been fixed meanwhile by the Crawlee team
• adds tests which verify that each exercise has its test :)

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Note

Low Risk
Test-only changes plus deletion of an unused exercise script; minimal production impact, with only a small risk of CI failures if tests are missing or external scraping outputs change.

Overview
Improves the academy webscraping exercise test suites by adding a new "covers all exercise scripts" check in both JS (*.mjs) and Python (*.py) that fails when any exercise file lacks a corresponding test invocation.

Removes the leftover wikipedia_calling_codes.mjs exercise file, and updates the Python Crawlee Netflix ratings test to assert an exact dataset size (-eq 5) instead of the prior workaround.

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