feat(sensors/gps): move GPS antenna offsets to per-receiver parameters (#26634)

* sensors: move GPS antenna offsets to per-receiver parameters

Move antenna position configuration from the single EKF2_GPS_POS_X/Y/Z
parameter set into per-receiver SENS_GPS{0,1}_OFF{X,Y,Z} parameters in
the sensors module. Each offset slot is matched to a physical receiver
by device ID (SENS_GPS{0,1}_ID), falling back to uORB instance index
when no IDs are configured.

The antenna offset is now carried through the SensorGps uORB message
and blended alongside other GPS states when multi-receiver blending is
active, so EKF2 receives the correct lever arm for whichever receiver
(or weighted combination) is selected.

- Add antenna_offset_{x,y,z} fields to SensorGps.msg
- Remove EKF2_GPS_POS_X/Y/Z params; EKF2 reads offset from gnssSample
- Add SENS_GPS{0,1}_ID and SENS_GPS{0,1}_OFF{X,Y,Z} params (module.yaml)
- Blend antenna offsets in GpsBlending (weighted average)
- Add unit tests for single, blended, and failover antenna offset cases
- Migrate params.c to module.yaml for the vehicle_gps_position module

* sensors: gps_blending: add asymmetric weight and fallthrough offset tests

Add two additional antenna offset test cases:

- dualReceiverAsymmetricWeightAntennaOffset: verify that unequal eph
  values produce correctly skewed blend weights (0.8/0.2) and that the
  output antenna offset reflects the weighted average
- blendingFallthroughAntennaOffset: verify that when blending is enabled
  but can_do_blending evaluates false (eph=0), the non-blending path
  correctly assigns the selected receiver's antenna offset

* feat(param_translation): translate EKF2_GPS_POS_ to SENS_GPS0_OFF_

* fix(msgs): proper formatting

* chore(msg): 0 if invalid/unknown

* fix(ROMFS): migrate EKF2_GPS_POS_ params

* fix(docs): migrate EKF2_GPS_POS_ params

* fix(blending): unsigned param

* Update msg/SensorGps.msg

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix(sensors/gps): remove 'values:' tag in  module.yaml

* fix(sensors/gps): unsigned instance index

* fix(blending): restore const on gps_blend_states()

Move antenna offset blending into blend_gps_data() where the
weights are computed, keeping gps_blend_states() const.

* fix(sensors/gps): fix msg annotation and restore SENS_GPS_PRIME values

Remove incorrect @invalid NaN annotation from antenna offset fields
(0.0 default is correct, not a sentinel). Restore values tag for
SENS_GPS_PRIME so QGC shows a dropdown.

* fix(gps_blending): fix pre-existing bug to clear _gps_updated flags

The loop iterates over i but always clears gps_select_index. The intent is to clear
all updated flags, but only the selected one gets cleared (N times)

* test(gps_blending): add stale update flag regression test

docs/en/dronecan/index.md

@@ -188,7 +188,7 @@ GPS CANNODE parameter ([set using QGC](#qgc-cannode-parameter-configuration)):
 
 Other PX4 Parameters:
 
-- If the GPS is not positioned at the vehicle centre of gravity you can account for the offset using [EKF2_GPS_POS_X](../advanced_config/parameter_reference.md#EKF2_GPS_POS_X), [EKF2_GPS_POS_Y](../advanced_config/parameter_reference.md#EKF2_GPS_POS_Y) and [EKF2_GPS_POS_Z](../advanced_config/parameter_reference.md#EKF2_GPS_POS_Z).
+- If the GPS is not positioned at the vehicle centre of gravity you can account for the offset using [SENS_GPS0_OFFX](../advanced_config/parameter_reference.md#SENS_GPS0_OFFX), [SENS_GPS0_OFFY](../advanced_config/parameter_reference.md#SENS_GPS0_OFFY) and [SENS_GPS0_OFFZ](../advanced_config/parameter_reference.md#SENS_GPS0_OFFZ).
 - If the GPS module provides yaw information, you can enable GPS yaw fusion by setting bit 3 of [EKF2_GPS_CTRL](../advanced_config/parameter_reference.md#EKF2_GPS_CTRL) to true.
 
 #### RTK GPS