test(cli): add comprehensive CLI integration tests (#8)

Add 18 CLI tests covering the full json2sql command surface:
- Basic convert commands (file, table name, dialects, output file)
- Flatten and schema-only options
- Stdin input and error cases (missing file, bad JSON, bad dialect)
- Version command
- Edge cases (array of objects, empty array, booleans, nulls)

Bumps CLI coverage from 0% to 80% and overall coverage from 73% to 89%.

Co-authored-by: DevForge Engineer <engineer@devforge.dev>

Author: Coding-Dev-Tools