common/utils: use decoder in findStateSensorId

Refactor findStateSensorId to read the fixed fields with
decode_pldm_platform_state_sensor_pdr(), replacing the packed struct
overlay and its manual offset math.

Tested: Build an image for the vr-nvl-obmc machine with the following
patches cherry-picked:
```
1. Kernel
   https://lore.kernel.org/all/20260702165524.2168091-1-jackyhuang@nvidia.com/
2. u-boot
   https://lore.kernel.org/openbmc/20260629105202.1417679-1-jackyhuang@nvidia.com/
3. meta-nvidia
   https://gerrit.openbmc.org/c/openbmc/openbmc/+/90959
   https://gerrit.openbmc.org/c/openbmc/openbmc/+/92346
4. platform-init
   https://gerrit.openbmc.org/c/openbmc/platform-init/+/92189
   https://gerrit.openbmc.org/c/openbmc/platform-init/+/92190
   https://gerrit.openbmc.org/c/openbmc/platform-init/+/92191
   https://gerrit.openbmc.org/c/openbmc/platform-init/+/92192
5. mctp-over-usb-config
   https://gerrit.openbmc.org/c/openbmc/dbus-sensors/+/80452
   https://gerrit.openbmc.org/c/openbmc/dbus-sensors/+/86236
   https://gerrit.openbmc.org/c/openbmc/dbus-sensors/+/86791
6. libpldm:
   https://gerrit.openbmc.org/c/openbmc/libpldm/+/90952
   https://gerrit.openbmc.org/c/openbmc/libpldm/+/91583
```

Built pldmd at this change and at master for vr-nvl-obmc. A pldmtool
GetPDR walk of the BMC repository and the FindStateSensorPDR D-Bus
lookups return output byte-identical to the unmodified build.

Change-Id: I3a9ae8c48a52afdc10deaff57f4bc7f119ac771b
Signed-off-by: JY Voon <jvoon@nvidia.com>