Pixhawk6X: emit 1Hz “hello world” STATUSTEXT and add targeted CI workflow

[Copilot]

This PR adds a Pixhawk6X-specific periodic GCS message so the flight controller sends hello world once per second. It also introduces a dedicated GitHub Actions workflow to automatically build the Pixhawk6X target for these changes.

• 1Hz GCS message on Pixhawk6X

  • Updated AP_Vehicle::one_Hz_update() to send STATUSTEXT at 1Hz only when building for Pixhawk6X (APJ_BOARD_ID == 53).
  • Scope is compile-time gated to avoid affecting other boards.

• New Pixhawk6X workflow

  • Added .github/workflows/test_pixhawk6x_hello.yml.
  • Triggers on push, pull_request, and workflow_dispatch, scoped to:
    • libraries/AP_Vehicle/**
    • .github/workflows/test_pixhawk6x_hello.yml
  • Builds ArduCopter for Pixhawk6X via ./waf configure --board Pixhawk6X + ./waf copter.
  • Uses least-privilege workflow token permissions (contents: read).

void AP_Vehicle::one_Hz_update(void)
{
    one_Hz_counter++;

#if defined(APJ_BOARD_ID) && (APJ_BOARD_ID == 53) && HAL_GCS_ENABLED
    GCS_SEND_TEXT(MAV_SEVERITY_INFO, "hello world");
#endif
    ...
}