Vast.ai: Support spot instances

[trashhalo]

Vast.ai offers interruptible (spot) instances, but the vastai backend has never been able to provision them.

Currently, get_offers_by_requirements() strips every spot offer:

# TODO(egor-s): spots currently not supported
extra_filter=lambda offer: not offer.instance.resources.spot,

and create_instance() never bids, so only on-demand instances are provisioned. gpuhunt already emits Vast spot offers (each on-demand offer is duplicated with price set to the machine's min_bid and spot=True), so these offers reach dstack today and are then discarded.

Vast's create endpoint is the same for both instance types. PUT /asks/{id}/ creates an interruptible instance when the payload includes a price (per-machine bid in $/hour), and an on-demand instance when it is omitted. This matches the official vast-python client, whose create instance --bid_price sends the same price field to the same endpoint.

The fix:

1. Drop the extra_filter that removed spot offers in get_offers_by_requirements(). Spot vs. on-demand selection is already driven by the run's spot_policy through the catalog query, so no offers are surfaced that the user did not ask for.
2. In run_job(), bid the spot offer's price (which gpuhunt already set to min_bid) and pass it to create_instance(); on-demand offers pass no bid.
3. Add the price field to the create_instance() payload (None for on-demand preserves the existing behavior).

Interruption handling requires no backend changes: the server infers a reclaimed spot instance from the lost runner connection and, because the offer is marked spot, terminates the job with INTERRUPTED_BY_NO_CAPACITY, which maps to the generic interruption retry event. This is the same path RunPod, OCI, and Nebius spot instances already use.

Tested against a live Vast.ai account: spot offers carry a min_bid below dph_base (e.g. an RTX 4090 at $0.24/hr on-demand vs. $0.12/hr min_bid), and the create endpoint accepts the price field. Unit tests cover that run_job() bids on spot offers and does not bid on on-demand offers.

Vast.ai API reference: https://docs.vast.ai/api/create-instance

AI assistance: This PR was written primarily by Claude Code (investigation, implementation, and tests) and reviewed by me before submitting.

[jvstme]

Hi @trashhalo. Thank you for contributing this useful feature. Overall, the PR looks good to me.

One issue I found during testing is that if you try to create an instance with an insufficient bid (for example, if someone outbids you just before you create the instance), Vast.ai returns an error but still creates an instance in the stopped state, which then continues to accumulate storage charges. This case requires additional cleanup.

I've already fixed this and added a few related improvements in 5e01c9c on dstackai:vastai-spot. If you agree with these changes, could you push the commits from dstackai:vastai-spot to Hanno-Labs:vastai-spot so they appear in this PR?

For some reason, GitHub won't let me push them to your repo myself, even though you've allowed edits from maintainers. This may be related to this known GitHub limitation.

[jvstme]

We'd like to include this feature in this week's release, so I'll merge as-is and add my fixes in a separate PR. Thanks again for contributing