Fix SpaceRuntime parsing after volumes mutation API calls

[abidlabs]

Not sure if this is a problem with Hub server or library, but basically I've noticed that PUT / DELETE /api/spaces/{repo_id}/volumes can return a JSON body that is not a full Space runtime document (e.g. only volume-related fields, no top-level stage). The client always did return SpaceRuntime(r.json()), which raises when stage is missing:

KeyError: 'stage'

Example of what goes wrong with the previous behavior:

from huggingface_hub import HfApi, Volume

api = HfApi()
# After a successful HTTP 200, the Hub may return e.g. {"volumes": [...]} without "stage".
# SpaceRuntime(data) expects data["stage"] → KeyError.
api.set_space_volumes(
    "org/space",
    [Volume(type="bucket", source="org/bucket", mount_path="/data")],
)

This PR checks more resiliently if the response is empty or does not contain "stage", in which case it falls back to get_space_runtime(repo_id) so callers still get a valid SpaceRuntime.

Noticed while testing the latest version of huggingface_hub (1.9.1) with trackio

---

Note

Low Risk
Low risk: change is localized to Space volume mutation return handling and adds a unit test; main behavioral difference is an extra get_space_runtime call when the Hub response is empty or not parseable as a full SpaceRuntime.

Overview
Fixes set_space_volumes and delete_space_volumes to be resilient to Hub responses that don’t include a full SpaceRuntime payload. A new helper (_space_runtime_after_mutation) now attempts to parse the response when possible, but falls back to get_space_runtime when the body is empty or missing required fields.

Adds a regression test ensuring that a non-runtime JSON response (e.g. only {"volumes": [...]}) triggers the fallback fetch instead of raising during SpaceRuntime construction.

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[davanstrien]

Thanks for the fix! IIUC the response parsing issue has been there since the volumes API was first added in #4018, but it wasn't reachable until #4054 fixed the request payload format.

[abidlabs]

yup @davanstrien exactly, thanks for that PR!