docs(contributing): fix 'uff' -> 'ruff' typos in lint commands

Both lint instructions in CONTRIBUTING.md had 'uff' instead of 'ruff',
making the commands non-functional for contributors.

test(converter): add missing coverage for unsupported root types and schema gen
- Test ValueError for string/number/boolean root types
- Test generate_schema with single dict root
- Test generate_schema with primitive root (falls back to 'value' column)
- Test generate_schema with flatten=True and nested arrays

Converter module coverage: 93% -> 100%

Author: DevForge Engineer

CONTRIBUTING.md

@@ -8,14 +8,14 @@ Thanks for your interest in contributing!
 2. Create a virtual environment: python -m venv .venv && source .venv/bin/activate
 3. Install dev dependencies: pip install -e ".[dev]"
 4. Run tests: pytest tests/ -v
-5. Lint: uff check src/
+5. Lint: `ruff check src/`
 
 ## Pull Requests
 
 - Fork the repo and create a feature branch
 - Add tests for any new functionality
 - Ensure all existing tests pass
-- Run uff check src/ --fix before committing
+- Run `ruff check src/ --fix` before committing
 - Keep PRs focused on a single change
 
 ## Reporting Issues