[Bug]: Verda spot offers marked NOT_AVAILABLE and not provisioned (e.g. B200 spot)

[IA386]

Steps to reproduce

On the verda backend, spot offers are incorrectly marked NOT_AVAILABLE whenever the on-demand variant of the same instance type is unavailable. Because provisioning requests offers with exclude_not_available=True, these spot offers are dropped and never provisioned. Observed with B200 spot instances.

Steps to reproduce

1. Configure a verda backend.
2. Provision a B200 spot instance, e.g. dstack apply -f task.dstack.yml --backend verda --gpu B200 --spot (B200 available as spot but not on-demand).
3. No offers are provisioned. In dstack offer --backend verda --gpu B200 --spot, B200 spot shows as "not available".

Root cause
VerdaCompute._get_offers_with_availability calls get_availabilities() without is_spot (returns on-demand inventory only) and keys the availability map by
(instance_name, region), ignoring spot. Spot offers therefore inherit on-demand availability.

Expected
Spot offers reflect spot inventory availability.

Actual behaviour

No response

Expected behaviour

No response

dstack version

0.20.22

Server logs

Additional information

No response