fix: GetGroupHandler: return 404 if group not found

[synackd]

Closes #84

Requesting a group that doesn't exist would return a 500 Internal Server Error, when a 404 Not Found would be more appropriate. This PR updates the code and the OpenAPI spec to reflect this change.

To test:

0. Start with clean cloud-init server.
1. ochami -L basic cloud-init group get config nonexistent
2. Output:

   ERROR | cloud_init-group-get.go:74 > cloud-init group request yielded unsuccessful HTTP response error="GetGroups(): failed to GET group from cloud-init: unsuccessful HTTP status: HTTP/2.0 404 Not Found: group (compute) not found\n"
   

3. In cloud-init logs:

   cloud-init-server[84700]: 2025/09/29 16:37:35 [cloud-init/m8u9Wq6ADy-000002] "GET http://demo.openchami.cluster:8443/admin/groups/compute HTTP/1.1" from 172.16.0.254 - 404 26B in 936.775µs
   cloud-init-server[84700]: 4:37PM INF Request bytes_in=0 bytes_out=26 duration=0.865364 method=GET remote_addr=172.16.0.254 request_id=cloud-init/m8u9Wq6ADy-000002 request_uri=/admin/groups/compute status="Not Found" status_code=404 user_agent=ochami/0.5.4
   

[alexlovelltroy]

LGTM