fix(bmp388): correct timestamp_sample to integration midpoint (#26920)

The BMP388 pressure measurement is integrated over a configurable
window (e.g. 37ms at 16x oversampling). The previous code used the
read time as timestamp_sample, which is the end of the integration
window. Correct to the midpoint by subtracting half the measurement
time, with a guard against unsigned underflow.

Author: dakejahl

src/drivers/barometer/bmp388/bmp388.cpp

@@ -171,8 +171,12 @@ BMP388::collect()
 
 	perf_begin(_sample_perf);
 
-	/* this should be fairly close to the end of the conversion, so the best approximation of the time */
-	const hrt_abstime timestamp_sample = hrt_absolute_time();
+	/* Correct for measurement integration delay: the pressure was
+	 * integrated over the preceding measurement_time window, so the
+	 * effective sample midpoint is half the measurement time before now. */
+	const hrt_abstime now = hrt_absolute_time();
+	const hrt_abstime half_meas = get_measurement_time() / 2;
+	const hrt_abstime timestamp_sample = (now > half_meas) ? (now - half_meas) : now;
 
 	if (!get_sensor_data(sensor_comp, &data)) {
 		perf_count(_comms_errors);