fix(cloud-proxy): surface Anthropic refusals instead of empty replies (#12424)

When an Anthropic model declines to answer, the Messages API returns
HTTP 200 with `stop_reason: "refusal"` and empty `content`. The
translate mode mapped that to a normal reply with no content, so the
OpenAI-compatible response looked like a successful completion
(`finish_reason: "stop"`, empty message). Routers, agents and UIs could
not tell "the model declined" from "the model had nothing to say", and
no fallback was triggered.

Return an explicit error for `stop_reason: "refusal"` in both the
non-streaming path and the streaming path (`message_delta`). Regular
replies, including empty `end_turn` replies, are unchanged.

Assisted-by: Claude:claude-opus-5-5

Signed-off-by: Stefan Walcz <stefan.walcz@walcz.de>

docs/content/operations/cloud-proxy.md

@@ -198,6 +198,8 @@ image blocks, and per-request usage tokens are dropped through the
 internal `Predict()` signature. Use passthrough mode when your clients need
 the upstream's full feature set.
 
+In translate mode, an Anthropic response with `stop_reason: "refusal"` is returned to the client as an error instead of an empty successful reply, for both non-streaming and streaming requests. A streamed response may already have delivered partial content when the refusal arrives. Responses that end normally (`end_turn`) are unaffected, even when their content is empty.
+
 #### Anthropic prompt caching
 
 `proxy.cache_prompt: true` makes the translator add Anthropic