Value actions use the wrong date format for Graylog-managed timestamp fields #27699

Description

The field value actions "Add to query" and "Insert into dashboard/search" format date values as ISO 8601 (YYYY-MM-DDTHH:mm:ss.SSSZ). This works for dynamically mapped date fields, but not for the date fields Graylog maps itself, which only accept uuuu-MM-dd HH:mm:ss.SSS (AbstractMapping.java, EventsIndexMapping.java).

Affected fields:

• Message indices: timestamp, gl2_receive_timestamp, gl2_original_timestamp, gl2_processing_timestamp
• Event indices: timestamp, timestamp_processing, timerange_start, timerange_end

The ISO format was introduced in #12244, which fixed the actions for dynamically mapped date fields.

Steps to reproduce

1. Search for any message.
2. Open the field value actions for timestamp and choose "Add to query".
3. Or: choose "Insert into dashboard/search" for a dashboard with a widget query like timestamp:$param$.

Expected behavior

The query contains the value in the field's mapping format, in UTC:

timestamp:"2026-10-06 07:36:15.116"

Actual behavior

• "Add to query": timestamp:"2026-10-06T07:36:15.116Z", which matches no messages.
• "Insert into dashboard/search": parameter value 2026-10-06T07:36:15.116Z, which matches no messages for these timestamp fields.