filter "home-assistant-device-registry": truncate ftms device identifiers
Device identifiers of the "ftms" domain (Fitness Machine Service, github.com/dudanov/hassio-ftms) contain the machine's full serial number in lowercase (e.g. ["ftms", "fs…"]).
Truncate them to 6 characters followed by "…", matching the length the filter already truncates the devices' "serial_number" attribute to, so both representations of the serial expose the same prefix.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>