test(integration): harden network block wait in health check tests

[rafaelwestphal]

Description

TestNetworkHealthCheck occasionally flakes (reporting false passes on blocked endpoints) because the deny-egress firewall rule propagation is eventually consistent.

The existing waitForNetworkBlock helper only polled a single telemetry endpoint (telemetry.googleapis.com) to determine if the block was active. However, because GCP firewall rules propagate to different IP ranges at slightly different times, other endpoints (like package repositories or download servers) could still be reachable when the health check started, causing the test to fail its assertions.

Solution
Hardened waitForNetworkBlock to poll all three critical endpoints that are verified by the health checks:

telemetry.googleapis.com (Telemetry API)
dl.google.com (Download/DL API)
packages.cloud.google.com (Packages API)
The helper now waits until all three endpoints are completely unreachable (all connections fail) before allowing the test to proceed. If any endpoint is still reachable, it continues polling, ensuring the firewall block has fully propagated across all relevant ranges.

Related issue

How has this been tested?

Checklist:

• Unit tests
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• Integration tests
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• Documentation
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• Minor version bump
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