fix(helm): propagate supervisor image overrides

[TaylorMutch]

Summary

Let the gateway use its version-pinned built-in supervisor image for the default Helm configuration while still propagating Helm image overrides into gateway.toml.

Related Issue

Follow-up to #2068 and #2070.

Changes

• Omit supervisor_image from the rendered gateway configuration when the standard repository and empty tag are unchanged.
• Propagate custom supervisor repositories and explicit tags to the Kubernetes driver settings.
• Resolve repository-only overrides with the effective gateway image tag and tag-only overrides with the official repository.
• Add Helm unit coverage for default, repository-only, tag-only, and complete overrides.
• Update chart, deployment, and architecture documentation.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (not applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[github-actions]

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[github-actions]

Label test:e2e applied for ecf67ce. Open Branch E2E Checks, find the run for commit ecf67ce, 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.

[TaylorMutch]

CC @benoitf as a followup to your PR

[benoitf]

Ok thanks @TaylorMutch