Crash when a lot of data needs to be proxied

# Problem Describing

Can't alloc os_mbuf when a lot of data needs to be proxied in BLE-MESH, so `om` is null to cause crash(assert).

```
static int proxy_send(uint16_t conn_handle, const void *data, uint16_t len) {
···
BT_DBG("%u bytes: %s", len, bt_hex(data, len));
om = ble_hs_mbuf_from_flat(data, len);
assert(om);
err = ble_gatts_notify_custom(conn_handle, svc_handles.proxy_data_out_h, om);
notify_complete();
···
}
```

# Related

These are also handled with assert: [pb_gatt_srv.c#L410-L411](https://github1s.com/apache/mynewt-nimble/blob/master/nimble/host/mesh/src/pb_gatt_srv.c#L410-L411) and [proxy_srv.c#L876-L877](https://github1s.com/apache/mynewt-nimble/blob/master/nimble/host/mesh/src/proxy_srv.c#L876-L877). We should reconsider how to deal with it.

Start with proxy_send in the BLE-MESH proxy path, then inspect the assert sites in pb_gatt_srv.c#L410-L411 and proxy_srv.c#L876-L877. Determine how allocation failure should be handled instead of asserting; done means these paths no longer crash when an os_mbuf cannot be allocated, with the behavior validated against the project’s existing checks.

Written by the indexing model from the issue text.