AP_ExternalAHRS: SBG: add SITL simulator, unit tests and autotest coverage

[tolesam]

Summary

Adds a simulated SBG INS (sbgECom protocol) to SITL, unit tests for the
SBG ExternalAHRS driver, and autotest coverage (Plane SBGEAHRS mission
test + SBG in the AHRS trim backend sweep). Also fixes two driver bugs
found while writing the tests.

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

Testing done:

• ./waf --targets tests/test_sbg && build/sitl/tests/test_sbg: 22/22 pass
• Tools/autotest/autotest.py build.Plane test.Plane.SBGEAHRS: passes
  (prearm, arm, takeoff, waypoints, landing)
• Manual SITL (--serial4=sim:SBG, EAHRS_TYPE=8, GPS1_TYPE=21,
  AHRS_EKF_TYPE=11): driver init and device info exchange OK, GPS 3D fix,
  position/attitude tracking home, SYS_STATUS GPS/baro/prearm bits stable
  at --speedup 1, 8 and 20

Description

One commit per subsystem:

• SITL: add simulated SBG INS — SIM_SBG SerialDevice sending
  sbgECom logs (IMU_SHORT, MAG, EKF_QUAT, EKF_NAV, AIR_DATA at 25/10Hz;
  GPS1_POS/GPS1_VEL/UTC_TIME at 10Hz) built from SITL flight dynamics,
  and answering SBG_ECOM_CMD_INFO. Frames are dropped whole when the
  buffer is full since partial frames desync the driver parser at high
  sim speedups; GNSS logs are sent first and at 10Hz because AP_GPS
  requires an average message delta below 215ms to report a healthy GPS;
  baro data gets noise so AP_Baro does not flag the sensor as stuck.
• HAL_SITL: hook up simulated SBG INS — attach with
  --serialN=sim:SBG.
• AP_HAL: add simulated SBG INS to SIMState — same hookup for
  sim-on-hardware, guarded by AP_SIM_EAHRS_SBG_ENABLED.
• AP_ExternalAHRS: SBG: open UART early, fix parser, add unit tests —
  the update thread waited for system init before opening the UART, but
  boot barometer calibration runs before that and only calibrates
  instances already producing data, so the external baro could never
  become healthy. Also fixes an off-by-one in parse_byte that dropped
  a payload of exactly SBG_PACKET_PAYLOAD_SIZE_MAX bytes,
  and makes the sbgECom struct header independent of
  AP_EXTERNAL_AHRS_SBG_ENABLED so the SITL simulator can
  reuse the definitions without compiling in the backend. Unit tests cover
  the packet parser state machine (CRC rejection/recovery, resync,
  oversized/truncated frames, full-size payload), frame sending and round
  trip, GPS week computation, EKF/GPS status decoding and the message
  copy helper, via a friend test class following the AP_GPS_NMEA pattern.
• autotest: add SBG external AHRS coverage — Plane SBGEAHRS
  mission test (75m final waypoint distance threshold) and SBG entry in the
  ExternalAHRS AHRS-trim backend configurations.