feat(kubernetes): support corporate upstream proxy

[loveRhythm1990]

Summary

Add Kubernetes support for the existing supervisor corporate HTTP forward-proxy path. This lets sandboxes in proxy-only clusters reach policy-approved TLS destinations through an operator-managed proxy without allowing workloads to choose, bypass, or observe the upstream route.

The Kubernetes driver previously had no configuration or Secret-projection path for the shared supervisor feature that Podman already uses. This PR adds that driver-level plumbing while preserving direct egress as the default when no proxy is configured.

Related Issue

Closes #2624

Changes

• Add Kubernetes driver configuration for https_proxy, no_proxy, proxy credential Secret name/key, insecure Basic-auth acknowledgement, and CONNECT-by-hostname mode. The URL remains intentionally limited to http:// forward proxies; HTTPS-to-proxy support is out of scope.
• Expose the configuration through gateway CLI/environment options and Helm upstreamProxy values, while keeping it deployment-owned rather than allowing per-sandbox driver_config overrides.
• Reuse the shared URL validation and enforce coherent settings at gateway startup: invalid URLs, unpaired Secret fields, invalid Secret names/keys, and unsafe credential settings fail closed.
• Project credentials as a read-only Kubernetes Secret volume, never as an environment value, annotation, or command-line value. The gateway validates the reference syntax; kubelet resolves Secret existence/key availability when the sandbox Pod starts; the supervisor validates the credential content and rejects malformed files.
• Require topology = \"sidecar\" whenever proxy credentials are configured. Combined topology shares a container filesystem with the workload, and workspace fsGroup can make an otherwise root-only Secret mount group-readable; rejecting this combination avoids a credential disclosure path. Credential-free proxy configuration can still use Combined topology.
• Pass upstream arguments only to the network-supervising container. In sidecar mode this is the network sidecar; network-init never receives proxy configuration and the agent container never receives the credential mount.
• Update the gateway reference, Kubernetes setup guide, compute-driver reference, architecture overview, Helm README, values, and rendering tests.

Testing

• mise run pre-commit passes
• Kubernetes driver unit tests and gateway TOML/Helm rendering tests updated
• Kubernetes mock-proxy e2e: authenticated CONNECT reaches a TLS upstream and policy-denied traffic never reaches the proxy
• Kubernetes mock-proxy e2e: NO_PROXY bypasses only the corporate proxy; policy-denied traffic remains denied
• Kubernetes e2e: missing credential Secret fails sandbox creation closed
• Kubernetes e2e: malformed credential content fails sandbox creation closed
• mise run test has one unrelated environment-dependent DNS failure: test_forward_public_ip_allowed_without_allowed_ips resolved dns.google to 198.18.1.240, which the SSRF guard correctly rejects. All Kubernetes proxy-specific checks above passed.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

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