[ci] update pre-commit versions, add some pixi things to config files

[jameslamb]

As I recommended in #6901 (review), this pulls some small CI / maintenance changes out of #6901. The main goal is to make the diff in #6901 a bit smaller so that proposal can be more thoughtfully and thoroughly reviewed, but these changes do have standalone value.

• updates all pre-commit hooks
• adds some pixi things to config files like .gitignore (in case anyone is using pixi in their local environment)
• fixes these new findings from ruff:

python-package/lightgbm/basic.py:290:42: PLW1508 Invalid type for environment variable default; expected `str` or `None`
python-package/lightgbm/basic.py:843:8: PLC1802 [*] `len(cat_cols)` used as condition without comparison
python-package/lightgbm/callback.py:77:5: PLC0415 `import` should be at the top-level of a file
python-package/lightgbm/compat.py:55:13: PLC0415 `import` should be at the top-level of a file
python-package/lightgbm/dask.py:772:38: C420 [*] Unnecessary dict comprehension for iterable; use `dict.fromkeys` instead
python-package/lightgbm/libpath.py:44:38: PLW1508 Invalid type for environment variable default; expected `str` or `None`
python-package/lightgbm/plotting.py:107:9: PLC0415 `import` should be at the top-level of a file
python-package/lightgbm/plotting.py:238:9: PLC0415 `import` should be at the top-level of a file
python-package/lightgbm/plotting.py:239:9: PLC0415 `import` should be at the top-level of a file
python-package/lightgbm/plotting.py:345:9: PLC0415 `import` should be at the top-level of a file
python-package/lightgbm/plotting.py:471:9: PLC0415 `import` should be at the top-level of a file
python-package/lightgbm/plotting.py:815:9: PLC0415 `import` should be at the top-level of a file
python-package/lightgbm/plotting.py:816:9: PLC0415 `import` should be at the top-level of a file
tests/python_package_test/test_sklearn.py:1733:5: PT031 `pytest.warns()` block should contain a single simple statement

[StrikerRUS]

LGTM, thank you!

[jameslamb]

Thank you!