test: fix unit tests for csv.ts (csvCell and toCsv)

[yachikadev]

Summary

Fixes the existing unit tests in test/csv.test.ts for CSV serialisation
utilities exported from src/lib/csv.ts.

Closes #2804

Changes Made

• Fixed incorrect expectation in toCsv single-row test case
  • Previous: expected header row only ("name,age")
  • Fixed: expects header + data row ("name,age\nAlice,30") which matches
    actual implementation behaviour

Why

The existing test assumed toCsv with a single row returns only headers.
But the implementation always appends data rows — even for a single row.
The wrong expectation would have caused CI to fail.

Test Coverage (already present, now correct)

All requirements from issue #2804 are covered:

• csvCell: null/undefined, plain text, comma, double-quote, newline, carriage return
• toCsv: empty array, single row, multiple rows, missing keys, numbers/booleans

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

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• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

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• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

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Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[yachikadev]

The Playwright smoke test failure is not related to this PR.
It is caused by a pnpm-lock.yaml lockfile mismatch with the current overrides config in package.json — a pre-existing repo issue. All checks relevant to this PR pass successfully.