sql/jsonpath: unwrap array with root key for index acceleration

Fixes: #156741

Previously, we didn't consider the case where the queried JSON is an array
and the root key can unwrap one layer of the array.

For example:

```
INSERT INTO json_tab VALUES (1,'[{"b": {"x": "y"}, "a": "x"}, null]');
CREATE INVERTED INDEX foo_inv ON json_tab(b);
SELECT a FROM json_tab@foo_inv WHERE jsonb_path_exists(b, '$.b');
```

This previously returned an empty result but should return 1.

This commit fixes the issue by adding array-unwrapping for the root key (`$`).

Release note (bug fix): Fix JSON path index acceleration with JSON array
objects. Only 25.4.0 releases are affected.

Author: ZhouXing19

pkg/sql/logictest/testdata/logic_test/jsonb_path_exists_index_acceleration

@@ -557,6 +557,31 @@ SELECT a FROM json_tab@primary WHERE jsonb_path_exists(b, '$.a ? (@.b == $x)', '
 statement error index "foo_inv" is inverted and cannot be used for this query
 SELECT a FROM json_tab@foo_inv WHERE jsonb_path_exists(b, '$.a ? (@.b == $x)', '{"x": "c"}') ORDER BY a;
 
+statement ok
+INSERT INTO json_tab VALUES
+(20,'[{"b": {"x": "y"}, "a": "x"}, null]'),
+(21,'[[{"b": {"x": "y"}, "a": "x"}], null]');
+
+query I
+SELECT a FROM json_tab@primary WHERE jsonb_path_exists(b, '$."b"[*]');
+----
+20
+
+query I
+SELECT a FROM json_tab@foo_inv WHERE jsonb_path_exists(b, '$.b');
+----
+20
+
+query I
+SELECT a FROM json_tab@primary WHERE jsonb_path_exists(b, '$."b"[*]');
+----
+20
+
+query I
+SELECT a FROM json_tab@foo_inv WHERE jsonb_path_exists(b, '$."b"[*]');
+----
+20
+
 subtest anykey
 
 statement ok