fix: add zarr3 to ImageFormat literal

COSEM API now returns format: zarr3 for some images, which the
pydantic schema rejected as an invalid literal. Hit this while
fetching jrc_hela-3 / er-mem_pred to validate #124 against real data.

Author: parallelArchitect

src/microsim/cosem/models.py

@@ -38,6 +38,7 @@
 ImageFormat = Literal[
     "n5",
     "zarr",
+    "zarr3",
     "precomputed",
     "neuroglancer_precomputed",
     "neuroglancer_multilod_draco",