Fix reorder_columns dropping client-specific fields from output

reorder_columns() was only keeping columns listed in FIELD_FORMAT_MAPPING,
silently discarding any extra columns like BCBS OFFSET_TERM/OFFSET_INDICATOR
and Clover's 12 full_context fields. The function's docstring documented
appending extra columns but the implementation was missing that step.

src/pipelines/shared/postprocessing/postprocessing_funcs.py

@@ -511,6 +511,10 @@ def reorder_columns(
     # Reorder columns to match column_order
     final_column_order = [col for col in column_order if col in df_copy.columns]
 
+    # Append any extra columns not in column_order (e.g. client-specific fields)
+    extra_columns = [col for col in df_copy.columns if col not in column_order]
+    final_column_order.extend(extra_columns)
+
     # Return the DataFrame with reordered columns
     return df_copy[final_column_order]