Fix malformed schema traversal to report validation errors instead of…

[p1c2u]

… internal exceptions

I reproduced this and implemented a fix on master branch locally.
Root cause:

• SchemaValidator recursively traversed schema nodes assuming mapping-like behavior.
• For malformed nodes like properties: {name: "string"}, that assumption could trigger internal exceptions (or produce poor UX depending on dependency versions), and in some cases even miss the error.
  What was changed:
• In openapi_spec_validator/validation/keywords.py, SchemaValidator.call now validates each schema node with check_schema(...) before descending into keywords like allOf / properties.
• If a node is malformed, we now yield OpenAPIValidationError and stop traversing that node.
• This avoids internal AttributeError paths and consistently reports a proper validation error.
  Tests added:
• tests/integration/validation/test_exceptions.py
  • Added regression for malformed property schema (name: "string") and assert it emits OpenAPIValidationError.
• tests/integration/test_main.py
  • Added CLI regression using stdin for the same malformed input and assert output is Validation Error (and not OK).

Fixes #436