Llama-stack integration is not compatible with `llama-stack>=0.4.0`

[anakin87]

Tests for the Llama-stack integration have been failing for a few days: https://github.com/deepset-ai/haystack-core-integrations/actions/runs/20904352674/job/60055198562

The recent v0.4.0 release contains several breaking changes that could be related.

[sjrl]

Should we just pin the earlier version of the library for now?

[anakin87]

Ah, yes. Done in #2715