revert the code refactor in loader.py, I will use a seperate pr for this refactor

Author: shuoweil

bigframes/session/loader.py

@@ -45,6 +45,7 @@
 import google.cloud.bigquery.table
 from google.cloud.bigquery_storage_v1 import types as bq_storage_types
 import pandas
+import pyarrow as pa
 
 import bigframes._tools
 import bigframes._tools.strings
@@ -1306,6 +1307,22 @@ def _transform_read_gbq_configuration(configuration: Optional[dict]) -> dict:
     return configuration
 
 
+def _has_json_arrow_type(arrow_type: pa.DataType) -> bool:
+    """
+    Searches recursively for JSON array type within a PyArrow DataType.
+    """
+    if arrow_type == bigframes.dtypes.JSON_ARROW_TYPE:
+        return True
+    if pa.types.is_list(arrow_type):
+        return _has_json_arrow_type(arrow_type.value_type)
+    if pa.types.is_struct(arrow_type):
+        for i in range(arrow_type.num_fields):
+            if _has_json_arrow_type(arrow_type.field(i).type):
+                return True
+        return False
+    return False
+
+
 def _validate_dtype_can_load(name: str, column_type: bigframes.dtypes.Dtype):
     """
     Determines whether a datatype is supported by bq load jobs.
@@ -1322,9 +1339,9 @@ def _validate_dtype_can_load(name: str, column_type: bigframes.dtypes.Dtype):
     if column_type == bigframes.dtypes.JSON_DTYPE:
         return
 
-    if isinstance(
-        column_type, pandas.ArrowDtype
-    ) and bigframes.dtypes.contains_db_dtypes_json_dtype(column_type):
+    if isinstance(column_type, pandas.ArrowDtype) and _has_json_arrow_type(
+        column_type.pyarrow_dtype
+    ):
         raise NotImplementedError(
             f"Nested JSON types, found in column `{name}`: `{column_type}`', "
             f"are currently unsupported for upload. {constants.FEEDBACK_LINK}"