curriculum: add actionable assertion messages to pathlib checks #106

Motivation

Bare assertions report only AssertionError, which does not tell a beginner what behavior was expected. Each hidden check should fail with a short, actionable message without changing what the exercise tests.

Current evidence

An exhaustive AST audit on dev at e1d4dae37234b75124bd6a53f06734296ebbbdee found 10 assert nodes without messages in this batch:

• checks/pathlib/pathlib1.py: 2
• checks/pathlib/pathlib2.py: 1
• checks/pathlib/pathlib3.py: 3
• checks/pathlib/pathlib4.py: 1
• checks/pathlib/pathlib5.py: 1
• checks/pathlib/pathlib6.py: 2

Reproduce from the repository root:

import ast
from pathlib import Path

files = [
    'checks/pathlib/pathlib1.py',
+    'checks/pathlib/pathlib2.py',
+    'checks/pathlib/pathlib3.py',
+    'checks/pathlib/pathlib4.py',
+    'checks/pathlib/pathlib5.py',
+    'checks/pathlib/pathlib6.py'
]
for filename in files:
    tree = ast.parse(Path(filename).read_text(encoding="utf-8"))
    missing = sum(
        isinstance(node, ast.Assert) and node.msg is None
        for node in ast.walk(tree)
    )
    print(filename, missing)

The full audit found 270 missing messages across 121 check files. This ticket is limited to 6 files and 10 messages.

Exact scope

Add a non-empty, beginner-facing message to each currently bare assertion in the listed files. Preserve every assertion predicate, execution order, helper statement, and success print.

Acceptance checklist

• All 10 currently bare assertions in the listed files have non-empty messages.
• Each message states the expected behavior or value in terms a learner can act on.
• Messages include the actual value when it is readily available and useful.
• Assertion predicates, ordering, side effects, and success output are unchanged.
• No check outside the listed files is changed.
• All 292 reference answers continue to pass.
• Changed check code remains Python 3.9 compatible.

Explicit exclusions

Do not change learner exercises, reference solutions, solutions/_answers.py, hints, documentation URLs, manifest order, exercise names, or curriculum semantics. Difficulty sequencing remains in #17, and contribution-guide wording remains in #39.

Likely files

• checks/pathlib/pathlib1.py
• checks/pathlib/pathlib2.py
• checks/pathlib/pathlib3.py
• checks/pathlib/pathlib4.py
• checks/pathlib/pathlib5.py
• checks/pathlib/pathlib6.py

Verification

python -m pytest tests/integration/test_solution_verify.py -q
python -m pytest -q
pythonlings --root tests/fixtures/passing_curriculum verify

Rerun the AST probe above and confirm every listed count is zero.

Sprint coordination

See Discussion #37 and tracker #52. This issue is intentionally unassigned. A coordination comment is welcome, but it does not reserve the issue. The first pull request that satisfies every acceptance criterion and verification requirement is considered for merge.