fix(devcontainer): bump to v3 Python image to fix Yarn GPG key failure (#781)

The v2 base image (mcr.microsoft.com/devcontainers/python:2-3.14-trixie)
ships with a Yarn Classic APT repository whose GPG key has expired,
causing apt-get update to fail during feature installation and breaking
the entire devcontainer build.

Bump to the v3 image (python:3-3.14-trixie, published 2026-01-30) which
no longer includes the Yarn APT repo, resolving the issue without any
additional workarounds.

Ref: devcontainers/images#1797

Author: PureWeen

.devcontainer/devcontainer.json

@@ -3,7 +3,7 @@
 {
   "name": "Python 3",
   // Or use a Dockerfile or Docker Compose file. More info: https://containers.dev/guide/dockerfile
-  "image": "mcr.microsoft.com/devcontainers/python:2-3.14-trixie",
+  "image": "mcr.microsoft.com/devcontainers/python:3-3.14-trixie",
   "features": {
     "ghcr.io/devcontainers/features/copilot-cli:1": {},
     "ghcr.io/devcontainers/features/github-cli:1": {},