(chore): add zarr v2 test

.azure-pipelines.yml

@@ -7,6 +7,7 @@ variables:
   PYTEST_ADDOPTS: --color=yes --junitxml=test-data/test-results.xml
   DEPENDENCIES_VERSION: "latest" # |"pre-release" | "minimum-version"
   TEST_TYPE: "standard" # | "coverage"
+  ZARR_VERSION: "3"
 
 jobs:
   - job: PyTest
@@ -28,6 +29,10 @@ jobs:
           python.version: "3.10"
           DEPENDENCIES_VERSION: "minimum"
           TEST_TYPE: "coverage"
+        zarr_v2:
+          python.version: "3.12"
+          ZARR_VERSION: "2"
+
     steps:
       - task: UsePythonVersion@0
         inputs:
@@ -49,6 +54,10 @@ jobs:
           path: $(uv_cache_dir)
         displayName: Cache pip packages
 
+      - script: echo "zarr < 3" >> ci/constraints.txt
+        displayName: "Install zarr v2"
+        condition: eq(variables['ZARR_VERSION'], '2')
+
       - script: uv pip install --system --compile "anndata[dev,test] @ ." -c ci/constraints.txt
         displayName: "Install dependencies"
         condition: eq(variables['DEPENDENCIES_VERSION'], 'latest')