feat(kubernetes): add combined topology config surface

[TaylorMutch]

Summary

Add the Kubernetes supervisor topology configuration surface with the existing combined topology as the only supported value. This keeps current sandbox behavior unchanged while creating the config, Helm, and documentation shape needed for future topology work.

Related Issue

References #1973.
References #1827.
References #981.
References #899.
References #1305.

Related PR: #2016.

Changes

• Add SupervisorTopology to the Kubernetes driver config and CLI/env parsing.
• Render supervisor_topology into Helm-managed gateway.toml with chart tests.
• Add supervisor.topology to Helm values and generated chart README.
• Add Kubernetes topology documentation for the supported combined mode and note that additional topologies are still being designed.
• Update gateway config and Kubernetes setup docs for the new field.

Testing

• mise run pre-commit passes
• cargo check -p openshell-driver-kubernetes passes
• cargo test -p openshell-driver-kubernetes --lib passes
• mise run helm:test passes
• E2E tests added/updated (not applicable; this PR preserves the existing combined topology behavior)

Checklist

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

[github-actions]

Label test:e2e applied for e7e3504. Open Branch E2E Checks, find the run for commit e7e3504, and click Re-run all jobs to execute with the label set. The run will execute the standard E2E suite after building the required gateway and supervisor images once. The matching required CI gate status on this PR will flip green automatically once the run finishes.

[github-actions]

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