Teach jsonpath string() to unwrap in lax mode

This was an ommission in commit 66ea94e, and brings it into compliance
with both other methods and the standard.

Per complaint from David Wheeler.

Author: David Wheeler, Jeevan Chalke
Reviewed-by: Chapman Flack

Discussion: https://postgr.es/m/A64AE04F-4410-42B7-A141-7A7349260F4D@justatheory.com

doc/src/sgml/func.sgml

@@ -17792,7 +17792,10 @@ ERROR:  jsonpath member accessor can only be applied to an object
     methods available in <type>jsonpath</type>.  Note that while the unary
     operators and methods can be applied to multiple values resulting from a
     preceding path step, the binary operators (addition etc.) can only be
-    applied to single values.
+    applied to single values.  In lax mode, methods applied to an array will be
+    executed for each value in the array.  The exceptions are
+    <literal>.type()</literal> and <literal>.size()</literal>, which apply to
+    the array itself.
    </para>
 
    <table id="functions-sqljson-op-table">

src/backend/utils/adt/jsonpath_exec.c

@@ -1606,6 +1606,9 @@ executeItemOptUnwrapTarget(JsonPathExecContext *cxt, JsonPathItem *jsp,
 				JsonbValue	jbv;
 				char	   *tmp = NULL;
 
+				if (unwrap && JsonbType(jb) == jbvArray)
+					return executeItemUnwrapTargetArray(cxt, jsp, jb, found, false);
+
 				switch (JsonbType(jb))
 				{
 					case jbvString:

src/test/regress/expected/jsonb_jsonpath.out

@@ -2525,7 +2525,10 @@ select jsonb_path_query('null', '$.string()', silent => true);
 (0 rows)
 
 select jsonb_path_query('[]', '$.string()');
-ERROR:  jsonpath item method .string() can only be applied to a bool, string, numeric, or datetime value
+ jsonb_path_query 
+------------------
+(0 rows)
+
 select jsonb_path_query('[]', 'strict $.string()');
 ERROR:  jsonpath item method .string() can only be applied to a bool, string, numeric, or datetime value
 select jsonb_path_query('{}', '$.string()');
@@ -2576,6 +2579,13 @@ select jsonb_path_query('1234', '$.string().type()');
  "string"
 (1 row)
 
+select jsonb_path_query('[2, true]', '$.string()');
+ jsonb_path_query 
+------------------
+ "2"
+ "true"
+(2 rows)
+
 select jsonb_path_query('"2023-08-15 12:34:56 +5:30"', '$.timestamp().string()');
 ERROR:  cannot convert value from timestamptz to timestamp without time zone usage
 HINT:  Use *_tz() function for time zone support.

src/test/regress/sql/jsonb_jsonpath.sql

@@ -586,6 +586,7 @@ select jsonb_path_query('"1.23aaa"', '$.string()');
 select jsonb_path_query('1234', '$.string()');
 select jsonb_path_query('true', '$.string()');
 select jsonb_path_query('1234', '$.string().type()');
+select jsonb_path_query('[2, true]', '$.string()');
 select jsonb_path_query('"2023-08-15 12:34:56 +5:30"', '$.timestamp().string()');
 select jsonb_path_query_tz('"2023-08-15 12:34:56 +5:30"', '$.timestamp().string()'); -- should work
 select jsonb_path_query_array('[1.23, "yes", false]', '$[*].string()');