Add proxy integration test

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Proxy Integration Test for upload-artifact

This test validates that the upload-artifact action properly honors http_proxy and https_proxy environment variables.

How it works

1. Environment Setup: Runs in an Ubuntu container with a Squid proxy service container
2. Traffic Blocking: Uses iptables to block all direct HTTP/HTTPS traffic (ports 80/443), allowing only traffic through the proxy
3. Verification: Confirms direct HTTPS requests fail while proxy requests succeed
4. Test Execution: Uploads a timestamped test artifact file

Key point

If the action ignores the proxy configuration and attempts direct connections, the upload will fail due to iptables blocking. Success indicates the action correctly routes traffic through the configured proxy.