Clean up test-generated artifacts tracked in git

[milton-minervino]

Unit tests were leaking build/test artifacts into the repo instead of using ephemeral storage.

• test_load_smartpredictor_1 wrote its round-trip pickle directly into the tracked tests/data/ directory, keyed by interpreter version (predictor_to_load_<pyver>.pkl) — every Python version bump left a new untracked file. Fixed to use tempfile.TemporaryDirectory(), matching the pattern already used by the neighboring TestSmartPredictorManifest test.
• Removed three now-dead pickled predictor fixtures (predictor_to_load_{38,39,310}.pkl) that were accidental commits from past interpreter upgrades and weren't read by any test.
• Widened the catboost_info/ gitignore pattern (was root-anchored /catboost_info/) so it also matches the catboost_info/ dumps CatBoost writes when executing tutorial notebooks (e.g. tutorial/generate_webapp/, tutorial/plots_and_charts/), not just one at the repo root.
• Removed a stray empty test file at the repo root.

No production code changes; tests/data/metadata.yaml, clean_titanic.pkl, and clean_titanic_pandas_3.pkl were confirmed to be genuine input fixtures (read-only, loaded by multiple tests) and left untouched. XGBoost/LightGBM were checked and confirmed not to write similar disk artifacts on .fit(), so no further gitignore changes were needed there.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

pytest: all tests passed, no leaked artifacts.

Test Configuration:

• OS: Linux
• Python version: 3.13.13
• Shapash version: 2.9.0