### What problem does this PR solve?
Fixes#15023
GPUStack is listed as unchecked in the Go-rewrite tracker #14736, and
`internal/service/llm.go:171` already classifies it as a self-deployed
provider alongside Ollama, Xinference, LocalAI, and LM Studio — but
`internal/entity/models/` had no `gpustack.go` driver, so the new Go API
server could not route GPUStack instances. This PR adds the chat surface
for GPUStack so it lines up with the existing self-hosted Go drivers.
Co-authored-by: Jin Hai <haijin.chn@gmail.com>
