test: add edge case test for non-YAML file in input/ without data_dir

[athola]

Context

PR #85 review suggestion #9: A .txt file inside input/ with data_dir=None falls through to ValueError. This code path is untested.

Current state

_infer_data_dir_and_name in src/simple_resume/shell/generate/core.py only handles YAML suffixes in the input/ detection path. Non-YAML files with no data_dir hit the generic ValueError branch.

Proposed change

Add a test in tests/unit/shell/test_infer_data_dir.py confirming that a non-YAML file in input/ without data_dir raises ValueError.

Acceptance criteria

• New test covering non-YAML file in input/ directory
• Test asserts ValueError is raised
• uv run pytest tests/unit/shell/test_infer_data_dir.py -v passes