fix: inject English system prompt for DeepSeek provider

DeepSeek models default to Chinese for some prompts. The ensureEnglish()
function prepends 'Always respond in English' as a system message when
no system prompt is already set. Applied to both ChatCompletion and
ChatCompletionStream paths.

internal/providers/helpers.go

@@ -445,3 +445,19 @@ func StreamGemini(ctx io.ReadCloser, ch chan<- *models.ChatCompletionStreamRespo
 
 	return nil
 }
+
+// ensureEnglish injects a system message instructing the model to respond in
+// English when no system prompt is already present. Some providers (e.g. DeepSeek)
+// default to Chinese for certain prompts.
+func ensureEnglish(req *models.UnifiedRequest) {
+	if len(req.Messages) > 0 && req.Messages[0].Role == "system" {
+		return // already has a system prompt, don't interfere
+	}
+	enMsg := models.UnifiedMessage{
+		Role: "system",
+		Content: []models.UnifiedContentPart{
+			{Type: "text", Text: "You are a helpful assistant. Always respond in English."},
+		},
+	}
+	req.Messages = append([]models.UnifiedMessage{enMsg}, req.Messages...)
+}