fix(ci): Use correct poetry command 'rest' instead of 'serve'

The backend pyproject.toml defines 'rest' as the script name, not 'serve'.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/platform-fullstack-ci.yml

@@ -127,7 +127,7 @@ jobs:
 
       - name: Start REST server in background
         run: |
-          poetry run serve &
+          poetry run rest &
           echo $! > /tmp/rest_server.pid
         working-directory: autogpt_platform/backend
         env: