feat(model): support BaseModel.load on local dirs without xturing.json

[glennko]

Summary:

• Extends BaseModel.load to accept model_name for local directories that do not contain xturing.json.
• Adds a clear error message when a local path is provided without xturing.json and without model_name.
• Refactors local model creation into a small helper to avoid duplicate logic.
• Adds unit tests for both success and error paths.

Validation:

• PYTHONPATH=src pytest -q tests/xturing/models/test_base_model_load.py

Closes #134