fix(curriculum): add actionable JSON assertion messages

[frobel0520]

Summary

• Add actionable, beginner-facing assertion messages to the seven bare checks in checks/json/json1.py through json6.py.
• Preserve every assertion predicate, execution order, helper statement, and success output.
• Keep the change limited to the six files listed in curriculum: add actionable assertion messages to JSON checks (batch 1 of 2) #101; no learner exercises, reference solutions, hints, documentation, or curriculum metadata changed.

Closes #101

Tests

• python -m pytest tests/integration/test_solution_verify.py -q — 1 passed
• python -m pytest -q — 209 passed, 6 failed
  • The six failures are Windows symlink-permission tests (WinError 1314) in doctor/manifest coverage; they are unrelated to this change.
• python -m pythonlings --root tests/fixtures/passing_curriculum verify — passed (passing1, passing2)
• AST audit of the six listed files — 0 bare assertions remaining

Screenshots

Not applicable; this is a curriculum check-message change with no TUI changes.

Checklist

• Updated docs when behavior changed — not applicable; curriculum: add actionable assertion messages to JSON checks (batch 1 of 2) #101 explicitly limits this change to assertion messages.
• Added or updated tests — not applicable; existing verification tests cover the unchanged predicates.
• Verified python -m pytest -q — completed; 209 passed and 6 unrelated Windows symlink-permission tests failed with WinError 1314.

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