fix(e2e): serialize Kubernetes HA deployment tests

[drew]

Summary

Serialize the Kubernetes HA E2E tests that mutate the same gateway deployment. Nextest launches each test in its own process, so the current in-process mutex cannot prevent one test from rolling the gateway while another is creating its sandbox.

Related Issue

Part of the #3583 stability campaign. The expanded Linux diagnostic run caught a gateway stream reset during the concurrent HA tests.

Changes

• Restrict the HA test binary to one Nextest test thread in the Kubernetes E2E wrapper.
• Document that deployment mutation tests require serial execution against a shared cluster.

Testing

• bash -n e2e/rust/e2e-kubernetes.sh.
• mise run pre-commit.
• mise run ci.
• Expanded GitHub Actions diagnostic for the combined stack tip.

Checklist

• Conventional Commit with DCO sign-off.
• Stacked with gh stack.

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[drew]

Closing to narrow the automated flake investigation to the four selected fixes (#3583/#3588, #3598, #3604, #3608). The findings remain available here for later follow-up.