test: add StickYaw unit test for unaided_yaw NaN transition bug (#26620)

* test: add StickYaw unit test for unaided_yaw NaN transition bug

Adds a functional gtest that verifies the yaw setpoint does not jump
discontinuously when unaided_yaw transitions from finite to NaN.
Reproduces the bug fixed in #25710.

* test(StickYaw): clarify unaided_yaw NaN root cause in test comment

NaN originates from corrupted IMU delta_angle propagating through
OutputPredictor::calculateOutputStates() Euler extraction.

src/lib/stick_yaw/CMakeLists.txt

@@ -35,3 +35,5 @@ px4_add_library(StickYaw
 	StickYaw.cpp
 )
 target_include_directories(StickYaw PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+
+px4_add_functional_gtest(SRC StickYawTest.cpp LINKLIBS StickYaw)

src/lib/stick_yaw/StickYawTest.cpp

@@ -0,0 +1,77 @@
+/****************************************************************************
+ *
+ *   Copyright (C) 2025 PX4 Development Team. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name PX4 nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+#include <gtest/gtest.h>
+#include "StickYaw.hpp"
+
+#include <px4_platform_common/defines.h>
+
+TEST(StickYawTest, UnaidedYawNanTransitionNoYawJump)
+{
+	// When unaided_yaw transitions from finite to NAN mid-flight,
+	// the yaw setpoint must not jump discontinuously.
+	// See: https://github.com/PX4/PX4-Autopilot/pull/25710
+
+	param_control_autosave(false);
+
+	StickYaw stick_yaw{nullptr};
+	float yawspeed_sp = 0.f;
+	float yaw_sp = NAN;
+	const float dt = 0.01f;
+
+	// Phase 1: Establish yaw lock at yaw=0 with unaided_yaw=0
+	stick_yaw.reset(0.f, 0.f);
+
+	for (int i = 0; i < 10; i++) {
+		stick_yaw.generateYawSetpoint(yawspeed_sp, yaw_sp, 0.f, 0.f, dt, 0.f);
+	}
+
+	EXPECT_NEAR(yaw_sp, 0.f, 0.01f);
+
+	// Phase 2: Simulate EKF yaw correction — yaw jumps to 0.3 rad while
+	// unaided_yaw stays at 0. This creates a yaw_error of 0.3 and triggers
+	// the convergence detector, building up _yaw_correction ≈ 0.3.
+	for (int i = 0; i < 5; i++) {
+		stick_yaw.generateYawSetpoint(yawspeed_sp, yaw_sp, 0.f, 0.3f, dt, 0.f);
+	}
+
+	const float yaw_sp_before = yaw_sp;
+	ASSERT_TRUE(PX4_ISFINITE(yaw_sp_before));
+
+	// Phase 3: unaided_yaw becomes NAN — yaw setpoint must not jump.
+	stick_yaw.generateYawSetpoint(yawspeed_sp, yaw_sp, 0.f, 0.3f, dt, NAN);
+
+	EXPECT_NEAR(yaw_sp, yaw_sp_before, 0.01f)
+			<< "Yaw setpoint jumped from " << yaw_sp_before << " to " << yaw_sp
+			<< " when unaided_yaw became NAN";
+}