Use Astral’s stable Python 3.15 Trixie slim image when available #3731

The Python 3.15 migration in #3730 uses ghcr.io/astral-sh/uv:0.13.0-trixie-slim, installs Python 3.15.0 explicitly, and installs Debian CA certificates. Astral currently lists Python 3.15 release-candidate images, and the stable Python 3.15 Trixie slim tags are not available.

Once Astral publishes a stable python3.15-trixie-slim image for both amd64 and arm64, switch src/mock_vws/_flask_server/Dockerfile to that image with a specific uv version pin. Verify that it supplies final Python 3.15 and working HTTPS certificates, then remove the explicit Python installation, its installation-directory settings, and the manual CA-certificate installation. Require the Docker build and deployment tests to pass on both architectures before removing the temporary setup.

Image documentation: https://docs.astral.sh/uv/guides/integration/docker/#available-images