Older Zed versions may send a buffer id of 0, which is no-longer
supported. (as of #6993)

This doesn't fix that, but it does ensure that we don't panic in the
workspace by maintaining the invariant that from_proto_state returns
Some(Task) if the variant matches.

It also converts the panic to an error should something similar happen
again in the future.


Release Notes:

- N/A