fix: handle quaternion type in serialize_solution

The imu_quat field in the solution dict contains a quaternion object
that is not JSON serializable, causing a crash when logging observations.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mrosseel

python/PiFinder/utils.py

@@ -39,6 +39,10 @@ def serialize_solution(solution: dict) -> str:
 
         if "numpy.float" in str(type(v)):
             v = float(v)
+
+        if "quaternion" in str(type(v)):
+            v = list(v)
+
         out_dict[k] = v
 
     return json.dumps(out_dict)