Fix Copilot setup to use uv instead of pip

The project uses uv for dependency management, but the Copilot setup
workflow was using actions/setup-python with pip caching. Since
`make install-dev` runs `uv sync`, the build fails because uv is not
installed. Replace with astral-sh/setup-uv to match lint-and-test.yml.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jsoucheiron

.github/workflows/copilot-setup-steps.yml

@@ -30,11 +30,13 @@ jobs:
         with:
           persist-credentials: false
 
-      - name: Setup Python
-        uses: actions/setup-python@83679a892e2d95755f2dac6acb0bfd1e9ac5d548 # v6.1.0
+      - name: Install uv
+        uses: astral-sh/setup-uv@0c5e2b8115b80b4c7c5ddf6ffdd634974642d182 # v5.4.1
         with:
-          python-version: "3.9"
-          cache: "pip"
+          enable-cache: true
+
+      - name: Setup Python
+        run: uv python install 3.9
 
       - name: Install development dependencies
         run: make install-dev