fix: remove default connection in csv reader causing hanging in multithreaded setups

Author: georgeRobertson

src/dve/core_engine/backends/implementations/duckdb/readers/csv.py

@@ -10,7 +10,6 @@
     DuckDBPyConnection,
     DuckDBPyRelation,
     StarExpression,
-    default_connection,
     read_csv,
 )
 from pydantic import BaseModel
@@ -61,7 +60,7 @@ def __init__(
         self.header = header
         self.delim = delim
         self.quotechar = quotechar
-        self._connection = connection if connection else default_connection
+        self._connection = connection if connection else ddb.connect(":memory:")
         self.field_check = field_check
         self.field_check_error_code = field_check_error_code
         self.field_check_error_message = field_check_error_message
@@ -181,7 +180,7 @@ def read_to_relation(  # pylint: disable=unused-argument
             ] + [pl.col(RECORD_INDEX_COLUMN_NAME)]
             df = df.select(pl_exprs)
 
-        return ddb.sql("SELECT * FROM df")
+        return self._connection.sql("SELECT * FROM df")
 
 
 class DuckDBCSVRepeatingHeaderReader(PolarsToDuckDBCSVReader):