fix(llama-cpp): do not stream the error text as content on pre-stream failures (#12425)

When the first result of a streamed request is an error (for example a
prompt that exceeds the context), PredictStream wrote the error message
as a Reply and only then returned the error status. LocalAI treated that
Reply as the first token: it sent the assistant role chunk and the error
text as `content` on an HTTP 200 stream. Because a chunk had already been
written, the pre-stream HTTP error path from #12204 never triggered, so
streaming clients still got a 200 with the error as model output, while
the same request without streaming correctly returns a 400.

Return the error only as the gRPC status. The e2e backend suite gets a
`context_overflow` capability (enabled for llama-cpp) that streams an
over-long prompt and asserts an error status with no content.

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

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

backend/cpp/llama-cpp/grpc-server.cpp

@@ -2176,10 +2176,10 @@ public:
             // connection is closed
             return grpc::Status(grpc::StatusCode::CANCELLED, "Request cancelled by client");
         } else if (first_result->is_error()) {
+            // Return the error only as the status. Writing it as a Reply first
+            // made it the first content chunk: LocalAI streamed the error text
+            // as assistant output on an HTTP 200 instead of failing the request.
             json error_json = first_result->to_json();
-            backend::Reply reply;
-            reply.set_message(error_json.value("message", ""));
-            writer->Write(reply);
             return grpc::Status(grpc::StatusCode::INTERNAL, error_json.value("message", "Error occurred"));
         }