improve: replace unit-style Typer CLI tests with integration smoke tests

[Coding-Dev-Tools]

Automated improvement by dev-engineer.

[github-actions]

🤖 Automated Code Review

✅ Ruff Lint — No issues

⚠️ Ruff Format — Formatting needed

Would reformat: src/api_contract_guardian/cli.py
Would reformat: src/api_contract_guardian/diff.py
Would reformat: src/api_contract_guardian/gate.py
Would reformat: src/api_contract_guardian/loader.py
Would reformat: src/api_contract_guardian/migration.py
Would reformat: tests/test_diff.py
Would reformat: tests/test_edge_cases.py
Would reformat: tests/test_gate.py
Would reformat: tests/test_migration.py
9 files would be reformatted, 6 files already formatted

✅ Secret Detection — Clean

✅ Large Files — Within limits

📊 Diff Stats — 1 file(s) changed

 tests/test_cli.py | 816 +++++++-----------------------------------------------
 1 file changed, 99 insertions(+), 717 deletions(-)

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Verdict: ⚠️ Warnings — Lint/format issues found. Recommend fixing before merge.

Automated by Coding-Dev-Tools/.github reusable workflow.