chore: move long running LLM tests to the large directory

Moved the following long-running tests from `tests/system/small/ml/` to `tests/system/large/ml/`:
- `test_llm_gemini_score`
- `test_llm_gemini_pro_score_params`
- `test_gemini_text_generator_predict_default_params_success`
- `test_gemini_text_generator_predict_output_schema_success`
- `test_gemini_text_generator_multi_cols_predict_success`
- `test_gemini_text_generator_predict_with_params_success`
- `test_create_load_gemini_text_generator_model`
- `test_gemini_text_generator_multimodal_input`
- `test_linear_reg_model_global_explain`

Author: google-labs-jules[bot]

tests/system/small/ml/test_llm.py

@@ -16,7 +16,6 @@
 from unittest import mock
 
 import pandas as pd
-import pyarrow as pa
 import pytest
 
 from bigframes import exceptions