Return correct status code for non existing sandbox in orchestrator in keep alive

[jakubno]

Note

Propagates gRPC NotFound to return API 404 when keeping a sandbox alive that no longer exists.

• Backend / Orchestrator:
  • keep_alive.go:
    • Map ErrSandboxNotFound from UpdateSandbox to API 404 Not Found during KeepAliveFor.
  • update_instance.go:
    • Inspect gRPC status; if codes.NotFound, return ErrSandboxNotFound.
    • Continue unwrapping and wrapping other errors as before.

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