ci(kubernetes): stabilize HA e2e setup

[TaylorMutch]

Summary

Stabilizes the HA Kubernetes e2e setup added after #1598 so the check can get through bare-runner setup and Helm install before running the Rust smoke.

Related Issue

Follow-up to #1598

Changes

• Sets the bare-runner Kubernetes e2e workflow to use MISE_VERSION=v2026.4.25, aligned with the project CI image
• Makes the mise version an input on the shared Kubernetes e2e reusable workflow
• Reuses .github/workflows/e2e-kubernetes-test.yml for both normal and HA Kubernetes e2e runs
• Removes the separate HA wrapper workflow; branch-e2e.yml now passes the HA values file directly to the shared workflow
• Uses mirror.gcr.io plus a pinned digest for the HA overlay's bundled Bitnami PostgreSQL image
• Enables Bitnami's non-standard image override only in the CI/dev HA overlay, because the mirror serves the same pinned image digest

Testing

• helm template openshell deploy/helm/openshell -n openshell -f deploy/helm/openshell/ci/values-skaffold.yaml -f deploy/helm/openshell/ci/values-high-availability.yaml --show-only charts/postgres/templates/primary/statefulset.yaml
• docker pull --platform linux/amd64 mirror.gcr.io/bitnami/postgresql@sha256:7651d7f24aad83fe68a222f7f20eded10d325c96ebee285ca5bf8162eddcba64
• mise run helm:lint
• git diff --check
• mise run pre-commit
• Local HA install with e2e/with-kube-gateway.sh true against a temporary k3d cluster using OPENSHELL_E2E_KUBE_EXTRA_VALUES=deploy/helm/openshell/ci/values-high-availability.yaml
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

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

[github-actions]

Label test:e2e-kubernetes applied for fa5e21c. Open the existing run and click Re-run all jobs to execute with the label set. The run will execute Kubernetes HA E2E after building the required gateway and supervisor images once. This is an optional proof-of-life suite; failures are visible in the workflow run but do not publish a required CI gate status.