bashunit learn generates starter files that are not valid bash

[Chemaclass]

Problem

bashunit learn writes a starter file for each lesson, and 7 of the 10 templates do not parse. A function body of only comments is a syntax error in bash, and the templates are full of them — 28 comment-only bodies across the lessons:

function tear_down() {
 # TODO: Clean up test files
}

So a learner who runs the lesson to generate the file, fills in one TODO and runs again gets:

challenge_test.sh: line 72: syntax error near unexpected token `}'
✗ Error: Source

That says nothing about the lesson, and the invalid file came from bashunit itself. It is the first thing a newcomer to the tool sees.

The fix needs two halves

Adding a : no-op to each body makes the file parse — and on its own that is worse, because an untouched template then passes the lesson:

template	run	result
comments only (today)	plain	syntax error
valid with :	plain	exit 0 — lesson completed with no work done
valid with :	--fail-on-risky	exit 1
actually solved	--fail-on-risky	exit 0

bashunit::learn::run_lesson_test runs "$BASHUNIT_ROOT_DIR/bashunit" "$test_file" --simple and treats exit 0 as success. A test with no assertions is risky, and risky exits 0 by default.

So: make the templates parse, and run the lesson's verification with --fail-on-risky. The learner then sees "risky: no assertions" — which is the actual state of their work — instead of a bash syntax error, and cannot complete a lesson without writing an assertion.

Verified

All 16 assertions the lessons teach exist in src/assert/, so the taught API itself is accurate; this is about the generated files.

[Chemaclass]

Done in #1257 (merged).