fix(k8s): respect template OPENSHELL_SANDBOX_COMMAND instead of overwriting

[Arnonrgo]

Summary

• apply_required_env() unconditionally overwrites OPENSHELL_SANDBOX_COMMAND with "sleep infinity"
• Custom sandbox images cannot specify their own entrypoint command
• Use upsert_env_if_absent() so template values take precedence

Changes

• Add upsert_env_if_absent() helper that only inserts if the env var doesn't already exist
• Change OPENSHELL_SANDBOX_COMMAND injection to use the new helper

Testing

• Unit test: existing value is preserved (not overwritten)
• Unit test: absent value gets default "sleep infinity"
• Unit test: full apply_required_env preserves template command
• cargo check -p openshell-driver-kubernetes passes

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