[fix] parser json.load PosixPath bug

[Belle0918]

What does this PR do?

Fix an AttributeError in read_args() when loading config from a JSON file.
json.load() expects a file-like object, but the code passes a PosixPath. Use Path.read_text() + json.loads() instead.

Fixes #10316

Before submitting

• Did you read the contributor guideline?
• Did you write any new necessary tests?

[gemini-code-assist]

Code Review

This pull request updates the JSON argument parsing logic in src/llamafactory/hparams/parser.py to correctly read file contents before parsing. A review comment suggests using OmegaConf.load() to simplify the implementation and improve consistency with the existing YAML loading logic.