Handle invalid YAML timestamps during scan

[KairosOps]

Fixes #584.

PyYAML raises a plain ValueError while constructing some malformed timestamps, for example 0000-01-01. The YAML transformer only converted yaml.YAMLError into ParsingError, so that constructor failure could escape the transformer and crash detect-secrets scan --all-files instead of falling back to the normal line scanner.

This treats that parse-time ValueError the same way as other YAML parsing failures. The transformer now raises ParsingError, get_transformed_file() ignores the failed structured transform, and scanning can continue over the raw file contents.

Checked:

• python -m py_compile detect_secrets\transformers\yaml.py tests\transformers\yaml_transformer_test.py tests\core\scan_test.py
• python -m pytest tests\transformers\yaml_transformer_test.py tests\core\scan_test.py -q
• python -m flake8 --select=E9,F63,F7,F82 detect_secrets\transformers\yaml.py tests\transformers\yaml_transformer_test.py tests\core\scan_test.py
• git diff --check

I also tried a wider nearby test set including tests/core/secrets_collection_test.py; that surfaced an unrelated existing assertion where test_data/config.md produced no detected secrets in this local environment. Full-file flake8 also reports many pre-existing E501 line-length violations in these files, so I kept the lint check scoped to syntax/undefined-name failures for this small patch.