Issue: Add RPMsg endpoint buffer size query API

Issue: Add RPMsg endpoint buffer size query API: a task in LegoFlow-SWE (Harbor dataset). Currently, applications using the RPMsg library have no portable way to determine the maximum buffer sizes supported by the underlying transport. To obtain the maximum message payload size, code must directly…

The task

Currently, applications using the RPMsg library have no portable way to determine the maximum buffer sizes supported by the underlying transport. To obtain the maximum message payload size, code must directly call transport‑specific functions (e.g., virtio‑specific helpers), which breaks API layering and renders…

Part of Lego-X/LegoFlow-SWE.