Narrower handling around unresolved refs

[p1c2u]

This pull request improves error handling in the handle_discriminator function and adds a new test to ensure correct behavior when unexpected reference errors occur. The main focus is on making sure only specific errors are caught and reported as validation errors, while others are properly propagated.

Error handling improvements:

• Updated handle_discriminator in openapi_schema_validator/_keywords.py to catch _WrappedReferencingError and check if the underlying cause is Unresolvable, yielding a ValidationError only in that case; other exceptions are re-raised to avoid swallowing unexpected errors.
• Added imports for _WrappedReferencingError and Unresolvable to support the improved error handling logic.

Testing enhancements:

• Added a new test test_discriminator_does_not_swallow_unexpected_ref_errors in tests/integration/test_validators.py to verify that unexpected errors (like RuntimeError) in _validate_reference are not swallowed by the discriminator logic.
• Imported Any and cast from typing to support monkeypatching in the new test.