feat: update array_to_string function to cast delimiter to string

Author: kosiew

python/datafusion/functions.py

@@ -317,7 +317,7 @@ def decode(expr: Expr, encoding: Expr) -> Expr:
 
 def array_to_string(expr: Expr, delimiter: Expr) -> Expr:
     """Converts each element to its text representation."""
-    return Expr(f.array_to_string(expr.expr, delimiter.expr.cast(pa.string())))
+    return Expr(f.array_to_string(expr.expr, delimiter.cast(str).expr))
 
 
 def array_join(expr: Expr, delimiter: Expr) -> Expr: