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.

[Copilot]

Pull request overview

This PR adds a GitHub Actions workflow to validate that the upload-artifact action respects http_proxy and https_proxy environment variables by forcing all HTTP(S) traffic through a Squid proxy.

Changes:

• Introduces .github/workflows/test-proxy.yml which runs a containerized Ubuntu job with a Squid proxy service.
• Sets up iptables rules to block direct outbound HTTP/HTTPS (ports 80/443) while allowing traffic only to the proxy and DNS.
• Verifies that direct HTTPS requests fail, HTTPS via the proxy succeeds, and then uploads a test artifact through the proxy using the local action (uses: ./).

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