chore: restrict internet access on sps

[aryamohanan]

SPS Network Restriction

1. Test containers run on an isolated Docker internal-net with no direct internet gateway.

2. Squid proxy runs separately on an egress-net, which has internet access.

3. The test container can reach the proxy, but cannot access the internet directly.

4. Squid allowlists only required domains such as npm, GitHub, Node.js, IBM IAM, and COS.

5. All other external requests are blocked by Squid.

6. Internal services such as LocalStack and other sidecars remain accessible through internal-net.

7. Added checks to verify:

   • Direct internet access is blocked.
   • Allowlisted domains work through the proxy.
   • Non-allowlisted domains are blocked.

[aryamohanan]

closing on favour of #2796