[Bug]: Offers show Available and Not available in the same GPU group #4441

Steps to reproduce

1. Open the Offers page in a project with a configured Verda backend.
2. Filter by backend verda, GPU name H100, and spot policy Spot.
3. Group by GPU.

This depends on current provider availability. In the reproduced case, Verda had two spot SKUs in FIN-01, both with one H100 80GB: 1H100.80S.30V was available, and 1H100.80S.32V was not available.

Actual behaviour

One H100 card shows Available, Not available. The available and unavailable SKUs are combined into the same group.

CPU cards also combine available and unavailable offers, with a shared price and resource range.

Expected behaviour

Available and unavailable offers appear in separate groups, each with its own prices and resource ranges. Only Available and Unknown may share a group.

dstack version

master dbe6d359f.

Notes

The initial GPU aggregation combines SKUs with the same GPU, count, spot policy and region without checking availability. The CPU aggregation does the same for identical CPU resources.

#4429 separates availability later, after those offers have already been combined, so it misses this case. #4440 includes availability in both initial aggregation keys.