Apply suggestions from code review of branch exposure-test

Co-authored-by: Richard Bowman <richard.bowman@cantab.net>

src/openflexure_microscope_server/things/camera/picamera_recalibrate_utils.py

@@ -260,8 +260,10 @@ def _test_exposure_settings(camera: Picamera2, percentile: float) -> _ExposureTe
 
     # A single request, to ensure metadata matches frame
     request = camera.capture_request()
+    try:
         metadata = request.get_metadata()
         image = request.make_array("raw")
+    finally:
         request.release()
     max_brightness = np.percentile(
         _channels_from_bayer_array(image),