Fix relative schema file paths on Windows

[WenJing95]

Fixes #2109.

Relative schema references such as ./schema.yaml receive a malformed UNC prefix on Windows because FileHandler builds a host prefix even when the URI has no host. Only add that prefix when a host is present, so relative references resolve from the working directory while UNC file URIs retain their host.

Changes proposed in this pull request:

• Cover relative references through FileHandler and resolve_refs.
• Verify absolute file URIs with escaped spaces and Windows UNC path conversion.

Validation on Windows 11 / Python 3.12.7:

• The two new relative-reference cases fail before the fix and pass afterward.
• pytest tests/test_references.py tests/test_operation2.py tests/test_json_validation.py -q: 42 passed.
• Black, isort, and flake8 checks pass for the changed files.

The existing test_api.py::test_relative_refs cases fail on both the base and this branch: their Windows drive path is used as a URI base, causing urljoin to discard the specification directory.