fix(ci): install podman-compose before running e2e-split

run_split.sh uses `podman compose` which delegates to the podman-compose
provider. The runner doesn't have it pre-installed, same situation as the
build-workspace job — fix by adding the same cache + pip install steps.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PcgXCGPQqN3fkehWdnfYE5
Signed-off-by: David Bensoussan <d.bensoussan@proton.me>

Author: Minipada

.github/workflows/ci.yaml

@@ -1033,6 +1033,16 @@ jobs:
         run: |
           echo "${{ secrets.GITHUB_TOKEN }}" | podman login ghcr.io -u "${{ github.actor }}" --password-stdin
 
+      - name: Cache podman-compose
+        uses: actions/cache@v4
+        with:
+          path: ~/.local
+          key: podman-compose-1.5.0
+      - name: Install podman-compose
+        run: |
+          pip install --user "podman-compose==1.5.0"
+          echo "$HOME/.local/bin" >> "$GITHUB_PATH"
+
       - name: Run split-topology E2E harness (unmanaged shipper + passthrough merge)
         # Proves merge_custom_config_files() (#472) works end-to-end: dc-ros writes a
         # single merged Vector config (rendered TOML + passthrough snippet) to the shared