Escape NUL bytes in white diff admin text.

[HyperSoWeak]

NUL is valid UTF-8, so backslashreplace leaves it as-is in the white diff
admin_text. PostgreSQL rejects NUL in text columns, so writing the
evaluation fails, and EvaluationService retries it forever, leaving the
testcase unscored. Triggered by a contestant printing a NUL byte on a
mismatching line (e.g. uninitialized memory) in a Batch task with the
built-in diff.

The fix escapes NUL as \x00 instead of stripping it, so the difference
stays visible.

Regression from b98e44b, not in any release. Similar to cms-dev#888. Hit this in
production during an exam.

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Reviewing files that changed from the base of the PR and between cc9dfaf and c7c81c3.

📒 Files selected for processing (2)

• cms/grading/steps/whitediff.py
• cmstestsuite/unit_tests/grading/steps/whitediff_test.py

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📝 Walkthrough

Walkthrough

_white_diff now escapes NUL characters in mismatch text as the literal \x00. A unit test checks that the mismatch is reported and the admin text contains no raw NUL byte.

Changes

Mismatch text escaping

Layer / File(s)	Summary
Escape NUL characters in mismatch output cms/grading/steps/whitediff.py, cmstestsuite/unit_tests/grading/steps/whitediff_test.py	_white_diff replaces decoded NUL characters in both displayed lines with the literal \x00. The test verifies that a NUL byte causes a mismatch and appears escaped in the admin text.

Priority: ⬆️ High

Estimated code review effort: 2 (Simple) | ~5 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to c7c81

NULs in mismatch diagnostics are displayed as visible text, and no merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: escaping NUL bytes in white diff admin text.

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[HyperSoWeak]

Superseded by cms-dev#1726.