fix(ci): add shell: bash to conditional install step for Windows

The bash [[ ]] syntax fails on Windows runners which default to
PowerShell. Explicitly setting shell: bash fixes the ParserError.

.github/workflows/ci.yaml

@@ -139,6 +139,7 @@ jobs:
           enable-cache: true
 
       - name: Install dependencies
+        shell: bash
         run: |
           if [[ "${{ matrix.python-version }}" == "3.9" || "${{ matrix.python-version }}" == "3.13" ]]; then
             uv sync --group tests