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/large/ml/test_linear_model.py

@@ -453,6 +453,7 @@ def test_model_centroids_with_custom_index(penguins_df_default_index):
     # If this line executes without errors, the model has correctly ignored the custom index columns
     model.predict(X_train.reset_index(drop=True))
 
+
 def test_linear_reg_model_global_explain(
     penguins_linear_model_w_global_explain, new_penguins_df
 ):

tests/system/large/ml/test_llm.py

@@ -16,7 +16,6 @@
 import pyarrow as pa
 import pytest
 
-from bigframes import exceptions
 from bigframes.ml import llm
 import bigframes.pandas as bpd
 from bigframes.testing import utils

tests/system/large/ml/test_multimodal_llm.py

@@ -12,8 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import pandas as pd
-import pyarrow as pa
 import pytest
 
 from bigframes.ml import llm