Add type infrastructure to know what reply is expected from each
`CommandToGsp`. Uses a marker type `NoReply` which does not implement
`MessageFromGsp` to mark commands which don't expect a response.

Update `send_command` to wait for a reply and add `send_command_no_wait`
which sends a command that has no reply, without blocking.

This prepares for adding locking to the queue.

Tested-by: Zhi Wang <zhiw@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
Link: https://patch.msgid.link/20260318-cmdq-locking-v5-3-18b37e3f9069@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

Add tests for continuation record splitting. They cover boundary
conditions at the split points to make sure the right number of
continuation records are made. They also check that the data
concatenated is correct.

Tested-by: Zhi Wang <zhiw@nvidia.com>
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
Link: https://patch.msgid.link/20260306-cmdq-continuation-v6-9-cc7b629200ee@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

Splits large RPCs if necessary and sends the remaining parts using
continuation records. RPCs that do not need continuation records
continue to write directly into the command buffer. Ones that do write
into a staging buffer first, so there is one copy.

Continuation record for receive is not necessary to support at the
moment because those replies do not need to be read and are currently
drained by retrying `receive_msg` on `ERANGE`.

Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
Link: https://patch.msgid.link/20260306-cmdq-continuation-v6-8-cc7b629200ee@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>