Validate and test stack inputs

Update picamera hardware test

src/openflexure_microscope_server/things/autofocus.py

@@ -21,7 +21,7 @@ from pydantic import BaseModel, computed_field, field_validator, model_validator
 import labthings_fastapi as lt
 from labthings_fastapi.types.numpy import NDArray
 
-from .camera import BaseCamera, CaptureParams
+from .camera import BaseCamera, CaptureParams, validate_capture_params
 from .stage import BaseStage
 
 LOGGER = logging.getLogger(__name__)
@@ -756,13 +756,19 @@ class AutofocusThing(lt.Thing):
         saving all images captured. No sharpness testing, restart
         logic, or autofocus is performed.
 
-        :param stack_parameters: SmartStackParams defining stack spacing,
-            image count, directory, and save resolution.
+        :param stack_parameters: StackParams defining stack spacing,
+            image count and backlash correction.
+        :param capture_parameters: CaptureParams defining save
+            resolution, images directory.
 
         :returns:
             - Final z position
             - List of z positions captured
         """
+        # Validate parameters and raise Exception if unsuitable
+        validate_stack_params(stack_parameters=stack_parameters)
+        validate_capture_params(capture_parameters=capture_parameters)
+
         captures: list[CaptureInfo] = []
         z_positions: list[int] = []
 
@@ -863,3 +869,24 @@ def _count_turning_points(sharpnesses: np.ndarray) -> int:
     d_sharpnesses = d_sharpnesses[prominent]
     # count the sign changes
     return int(np.sum(d_sharpnesses[1:] * d_sharpnesses[:-1] < 0))
+
+
+def validate_stack_params(stack_parameters: StackParams) -> None:
+    """Validate stack parameters for a z-stack acquisition.
+
+    Ensures that the parameters allow a physical stack, without negative or zero
+    values where they would cause crashes.
+
+    :param stack_parameters: StackParams object containing stacking settings.
+
+    :raises ValueError: If any stack_parameters are found to be unusable.
+    """
+    if stack_parameters.images_to_save <= 0:
+        raise ValueError(
+            f"Invalid number of images to save: {stack_parameters.images_to_save}. Must be > 0."
+        )
+
+    if stack_parameters.settling_time < 0:
+        raise ValueError(
+            f"Invalid settling time: {stack_parameters.settling_time}. Must be positive or 0."
+        )