feat: Add GitHub Actions workflow to test distributions on TestPyPI

- New workflow triggered on testpypi version tags
- Builds wheel and sdist
- Tests installation on Python 3.11, 3.12, 3.13
- Publishes to TestPyPI with twine check validation

Author: Claude Subagent

.github/workflows/testpypi.yml

@@ -0,0 +1,55 @@
+name: Publish distributions to TestPyPI
+
+on:
+  workflow_dispatch:
+  push:
+    tags:
+      - "v*-testpypi"
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+      
+      - name: Install build tools
+        run: python -m pip install --upgrade build twine
+      
+      - name: Build distributions
+        run: python -m build
+      
+      - name: Check distributions
+        run: twine check dist/*
+      
+      - name: Publish to TestPyPI
+        uses: pypa/gh-action-pypi-publish@release/v1
+        with:
+          repository-url: https://test.pypi.org/legacy/
+          password: ${{ secrets.TEST_PYPI_API_TOKEN }}
+          skip_existing: true
+
+  test-install:
+    needs: build
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.11", "3.12", "3.13"]
+    steps:
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+      
+      - name: Wait for TestPyPI package
+        run: sleep 30
+      
+      - name: Install from TestPyPI
+        run: |
+          pip install --index-url https://test.pypi.org/simple/ \
+            --extra-index-url https://pypi.org/simple/ \
+            zarr
+      
+      - name: Test import
+        run: python -c "import zarr; print(zarr.__version__)"

changes/3798.feature.md

@@ -0,0 +1 @@
+Add GitHub Actions workflow to test distributions on TestPyPI before release