Preserve tz when converting to jsonb timestamptz

The JSONB jbvDatetime type has a field for offset, and displays the time
in that offset. For example, when the time zone GUC is set to
America/New_York, the jsonpath `timestamp_tz()` method returns a value
that displays a parsed value with its offset, not the local offset:

    david=# set time zone 'America/New_York';
    SET
    david=# select jsonb_path_query_tz('"2024-08-15 12:34:56+10"', '$.timestamp_tz()');
        jsonb_path_query_tz
    -----------------------------
    "2024-08-15T12:34:56+10:00"

This was not true for values parsed by `timestamp_tz()` that lacked an
offset. It correctly assumes the local time zone, but displays it in
UTC:

    david=# select jsonb_path_query_tz('"2024-08-15 12:34:56"', '$.timestamp_tz()');
        jsonb_path_query_tz
    -----------------------------
    "2024-08-15T16:34:56+00:00"

To fix this inconsistent behavior, determine the offset for values being
cast from `DATEOID` and `DATEOID` types to `jpiTimestampTz` and store it
in the resulting jbvDatetime value. With this change, the result now
preserves the offset just as it does when converting from offset-aware
values:

    david=# select jsonb_path_query_tz('"2024-08-15 12:34:56"', '$.timestamp_tz()');
        jsonb_path_query_tz
    -----------------------------
    "2023-08-15T12:34:56-04:00"

Author: David Wheeler
Reviewed-by: Junwang Zhao
Discussion: https://postgr.es/m/7DE080CE-6D8C-4794-9BD1-7D9699172FAB%40justatheory.com

Author: theory

src/backend/utils/adt/jsonpath_exec.c

@@ -2707,12 +2707,21 @@ executeDateTimeMethod(JsonPathExecContext *cxt, JsonPathItem *jsp,
 			break;
 		case jpiTimestampTz:
 			{
+				struct pg_tm tm;
+				fsec_t		fsec;
 				/* Convert result type to timestamp with time zone */
 				switch (typid)
 				{
 					case DATEOID:
 						checkTimezoneIsUsedForCast(cxt->useTz,
 												   "date", "timestamptz");
+						DateADT dateVal = DatumGetDateADT(value);
+						j2date(dateVal + POSTGRES_EPOCH_JDATE,
+							&(tm.tm_year), &(tm.tm_mon), &(tm.tm_mday));
+						tm.tm_hour = 0;
+						tm.tm_min = 0;
+						tm.tm_sec = 0;
+						tz = DetermineTimeZoneOffset(&tm, session_timezone);
 						value = DirectFunctionCall1(date_timestamptz,
 													value);
 						break;
@@ -2726,6 +2735,9 @@ executeDateTimeMethod(JsonPathExecContext *cxt, JsonPathItem *jsp,
 					case TIMESTAMPOID:
 						checkTimezoneIsUsedForCast(cxt->useTz,
 												   "timestamp", "timestamptz");
+						if (timestamp2tm(DatumGetTimestamp(value), NULL, &tm, &fsec, NULL, NULL) == 0) {
+							tz = DetermineTimeZoneOffset(&tm, session_timezone);
+						}
 						value = DirectFunctionCall1(timestamp_timestamptz,
 													value);
 						break;

src/test/regress/expected/jsonb_jsonpath.out

@@ -2964,7 +2964,7 @@ HINT:  Use *_tz() function for time zone support.
 select jsonb_path_query_tz('"2023-08-15"', '$.timestamp_tz()'); -- should work
      jsonb_path_query_tz     
 -----------------------------
- "2023-08-15T07:00:00+00:00"
+ "2023-08-15T00:00:00-07:00"
 (1 row)
 
 select jsonb_path_query('"12:34:56"', '$.timestamp_tz()');
@@ -3151,7 +3151,7 @@ HINT:  Use *_tz() function for time zone support.
 select jsonb_path_query_tz('"2023-08-15 12:34:56"', '$.timestamp_tz()'); -- should work
      jsonb_path_query_tz     
 -----------------------------
- "2023-08-15T02:34:56+00:00"
+ "2023-08-15T12:34:56+10:00"
 (1 row)
 
 select jsonb_path_query('"2023-08-15 12:34:56 +05:30"', '$.timestamp_tz()');