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/deepseek.go

@@ -62,6 +62,9 @@ func (u *deepSeekUsage) ToUnified() *models.Usage {
 }
 
 func (p *DeepSeekProvider) ChatCompletion(ctx context.Context, req *models.UnifiedRequest) (*models.ChatCompletionResponse, error) {
+	// Ensure English responses — DeepSeek defaults to Chinese for some prompts
+	ensureEnglish(req)
+
 	messagesJSON, err := MessagesToOpenAIJSON(req.Messages)
 	if err != nil {
 		return nil, fmt.Errorf("failed to convert messages: %w", err)
@@ -129,6 +132,8 @@ func (p *DeepSeekProvider) ChatCompletion(ctx context.Context, req *models.Unifi
 }
 
 func (p *DeepSeekProvider) ChatCompletionStream(ctx context.Context, req *models.UnifiedRequest) (<-chan *models.ChatCompletionStreamResponse, error) {
+	ensureEnglish(req)
+
 	messagesJSON, err := MessagesToOpenAIJSON(req.Messages)
 	if err != nil {
 		return nil, fmt.Errorf("failed to convert messages: %w", err)