builtins: array_to_string should traverse nested arrays

Fixes cockroachdb#95588

In Postgres, `array_to_string` traverses nested arrays and prints
their contents. In CRDB, the nested array structures are printed
out. For example,
`SELECT array_to_string(ARRAY[ARRAY[ARRAY[5,6], ARRAY[2,3]]], ' ');`

CRDB Result: `ARRAY[ARRAY[5:::INT8,6:::INT8],ARRAY[2:::INT8,3:::INT8]]`
Postgres Result: `5 6 2 3`

This fix brings the behavior of `array_to_string` in line with
Postgres, and avoids printing the nested ARRAY structures.

Some tools like GoldenGate rely on Postgres-compatible  behavior of
`array_to_string` for proper functioning.

Release note (bug fix): This patch fixes the array_to_string built-in
function so that nested arrays are traversed without printing 'ARRAY'
at each nesting level.

pkg/sql/logictest/testdata/logic_test/builtin_function

@@ -2480,6 +2480,18 @@ SELECT array_to_string(NULL, ','), array_to_string(NULL, 'foo', 'zerp')
 ----
 NULL  NULL
 
+# Builtin array_to_string should recursively search nested arrays.
+query T
+SELECT array_to_string(ARRAY[ARRAY[ARRAY[5,6], ARRAY[2,3]], ARRAY[ARRAY[7,8], ARRAY[4,44]]], ' ');
+----
+5 6 2 3 7 8 4 44
+
+# Builtin array_to_string should recursively search nested arrays.
+query T
+SELECT array_to_string(ARRAY[(SELECT ARRAY[1,2]::int2vector)],' ');
+----
+1 2
+
 subtest pg_is_in_recovery
 
 query B colnames

pkg/sql/sem/builtins/builtins.go

@@ -8976,6 +8976,13 @@ func arrayToString(
 	evalCtx *tree.EvalContext, arr *tree.DArray, delim string, nullStr *string,
 ) (tree.Datum, error) {
 	f := evalCtx.FmtCtx(tree.FmtArrayToString)
+	arrayToStringHelper(evalCtx, arr, delim, nullStr, f)
+	return tree.NewDString(f.CloseAndGetString()), nil
+}
+
+func arrayToStringHelper(
+	evalCtx *tree.EvalContext, arr *tree.DArray, delim string, nullStr *string, f *tree.FmtCtx,
+) {
 
 	for i := range arr.Array {
 		if arr.Array[i] == tree.DNull {
@@ -8984,13 +8991,17 @@ func arrayToString(
 			}
 			f.WriteString(*nullStr)
 		} else {
-			f.FormatNode(arr.Array[i])
+			if nestedArray, ok := arr.Array[i].(*tree.DArray); ok {
+				// "Unpack" nested arrays to be consistent with postgres.
+				arrayToStringHelper(evalCtx, nestedArray, delim, nullStr, f)
+			} else {
+				f.FormatNode(arr.Array[i])
+			}
 		}
 		if i < len(arr.Array)-1 {
 			f.WriteString(delim)
 		}
 	}
-	return tree.NewDString(f.CloseAndGetString()), nil
 }
 
 // encodeEscape implements the encode(..., 'escape') Postgres builtin. It's