fix: allow disabling LightGBM early stopping

[he-yufeng]

Summary

• skip LightGBM early_stopping() callback creation when early_stopping_rounds resolves to None
• apply the same guard to the high-frequency LightGBM wrapper
• add regression tests that assert the wrappers omit the early-stopping callback when it is disabled

Fixes #2226

To verify

• python -m py_compile qlib\contrib\model\gbdt.py qlib\contrib\model\highfreq_gdbt_model.py tests\model\test_lightgbm_early_stopping.py
• python -m black --check --target-version py313 --fast tests\model\test_lightgbm_early_stopping.py
• git diff --check

Local blocker

I tried python -m pytest tests\model\test_lightgbm_early_stopping.py -q, but this Windows checkout could not import qlib before collection because the Cython extension qlib.data._libs.rolling was not built. pip install -e . then failed while compiling that extension because MSVC could not find io.h from the local Python/Windows SDK setup. The test itself is included for CI where qlib's extension build is available.