backend: add tests for malformed dialogue pack metadata

Goal

Add regression coverage proving that malformed manifest.json content never crashes dialogue-pack listing.

Scope

• Work in apps/api/tests/test_api_contract.py.
• Import list_dialogue_packs from app.dialogue_match.
• Use pytest's tmp_path to create a small persona/version/manifest.json tree containing invalid JSON.
• Call list_dialogue_packs(tmp_path) and assert that listing completes without an exception.
• Assert that the returned pack keeps the directory-derived personaId and packVersion, while topics and levels fall back to empty lists and the three count fields fall back to 0.
• Keep the PR focused on this regression test. Change production code only if the new test exposes a real failure.

Acceptance Criteria

• Invalid manifest JSON does not crash pack listing.
• Safe default metadata is asserted explicitly.
• Existing valid dialogue-pack behavior remains covered and passing.
• The PR body includes Closes #21 and names the checks run.

Suggested Verification

cd apps/api
.venv/bin/python -m pytest tests/test_api_contract.py -k dialogue_pack
cd ../..
python3 scripts/check_public_tree.py

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