fix: remove unsupported vanilla cfg parameter

[primorLee]

Fixes #221

Summary

• Stop forwarding the obsolete dyn_thresh_config argument to VanillaCFG.
• Add a focused regression test for the default sampling configuration.

Problem

The default SamplingParams select VanillaCFG, whose constructor only accepts scale. The inference API also supplied dyn_thresh_config, causing get_sampler_config(SamplingParams()) to raise TypeError before model inference began.

Validation

• pytest -q tests/inference/test_inference.py -k default_sampling_params_build_a_vanilla_cfg_sampler — 1 passed, 72 deselected
• python -m py_compile sgm/inference/api.py tests/inference/test_inference.py
• git diff --check

The focused pytest used a local CPU dependency bootstrap for optional host dependencies. The full GPU/model suite was not run, and pre-commit was unavailable in the host environment.