[walcz-de]

Mistral's chat completions API types message.content and delta.content as
string | null | ContentChunk[] (AssistantMessage / DeltaMessage in
https://docs.mistral.ai/openapi.yaml). Reasoning models answer with a
ThinkChunk plus a TextChunk:

"content": [
  {"type": "thinking", "thinking": [{"type": "text", "text": "..."}]},
  {"type": "text", "text": "Hallo!"}
]

and, per https://docs.mistral.ai/capabilities/reasoning/, the stream sends
chunk arrays during thinking and plain strings once the answer starts.

Translate mode decoded content into a string, so every such response failed:

cloud-proxy: decode response: json: cannot unmarshal array into Go struct
field openAIMessage.choices.message.content of type string

Reproduction: a model with backend: cloud-proxy, proxy.mode: translate,
provider: openai, upstream_url: https://api.mistral.ai/v1/chat/completions
and a Mistral reasoning model as upstream_model; any chat request fails
with the error above.

Change:

• Decode content as a string or an array of parts. text parts become the
  answer, thinking parts go to ChatDelta.ReasoningContent, other part
  types are ignored. Requests still send content as a string.
• Streaming deltas get the same handling, so reasoning streams as
  reasoning_content.
• When a non-streaming reply carries a ChatDelta (tool calls or reasoning),
  the delta now also holds the answer text, because consumers take the answer
  from the delta when one is present. Plain string content is unchanged.

Tests: new Ginkgo specs in provider_openai_content_test.go with a fake
upstream returning Mistral-shaped bodies (non-streaming, tool calls,
streaming across both shapes) plus counter-checks for plain string content
and deltas. go test ./... in backend/go/cloud-proxy passes.

Docs: docs/content/operations/cloud-proxy.md (translate mode section).

Breaking changes: none for plain string content. Behaviour change: a non-streaming reply that carries tool calls or reasoning now keeps its answer text in the ChatDelta (previously dropped).

AI assistance: prepared with Claude Code (see the Assisted-by trailer); reviewed and signed off by me.