fix(ekf2): fix observation_variance unit mismatch in comparison

[AkaiEurus]

Solved Problem

This PR fixes a units mismatch in Ekf::controlGnssYawEstimator().
Previously, the code used aid_src_vel.observation_variance[0] (vel_var, in m^2/s^2) as if it were a 1-sigma speed accuracy (m/s):

• compared directly against EKF2_REQ_SACC (m/s)
• passed directly to EKFGSF_yaw::fuseVelocity(), which expects 1-sigma accuracy (m/s)

Changelog Entry

For release notes:

Bugfix: fix observation_variance unit mismatch in comparison

[Copilot]

Pull request overview

Fixes a GNSS velocity units mismatch in Ekf::controlGnssYawEstimator() so the EKF-GSF yaw estimator receives and validates GNSS speed accuracy in the correct units (m/s) instead of variance (m²/s²).

Changes:

• Convert GNSS velocity observation variance to 1-sigma velocity accuracy using sqrtf().
• Compare 1-sigma accuracy against EKF2_REQ_SACC (m/s) instead of comparing variance to a speed threshold.
• Pass 1-sigma accuracy (m/s) into EKFGSF_yaw::fuseVelocity() as expected by its API.