Fix memmory availability check for embedding generation

[Ngalstyan4]

There may be an issue with the way we check memory availability from rust for embedding generation.
If all of RAM is filled with cache that can be swapped out upon memory pressure, I think it still reports low memory error

postgres=# select text_embedding('BAAI/bge-small-en', 'haha');
ERROR:  Not enough free memory to run the model. Memory needed: 137.70MB, Memory available: 53.87MB

note that available memory below is much larger than free memory

n@cb28478e2f23:/$ free -h
               total        used        free      shared  buff/cache   available
Mem:           125Gi       1.4Gi       1.1Gi        93Gi       123Gi        29Gi