Tracks #3776, which carries forward the optional Vertex AI bundle work from PR #2910 after the original fork branch could not be updated.\n\nScope:\n- keep google-cloud-aiplatform as the openhands-sdk[vertex] optional extra\n- preserve the lean default agent-server build with Vertex as an opt-in bundle\n- resolve conflicts against current main and apply review readability feedback
Tracks #3776, which carries forward the optional Vertex AI bundle work from PR #2910 after the original fork branch could not be updated.\n\nScope:\n- keep google-cloud-aiplatform as the openhands-sdk[vertex] optional extra\n- preserve the lean default agent-server build with Vertex as an opt-in bundle\n- resolve conflicts against current main and apply review readability feedback