fix: correct deepseek pricing, gemini streaming tokens, and group-name logging

- Add promo discount system for deepseek-v4-pro (75% off until 2026-05-31)
- Rewrite StreamGemini to handle both SSE and JSON array response formats,
  fixing 0-token logging for gemini-3-flash and gemini-3-flash-preview
- Fall back to model group name for cost lookup when concrete model
  isnt in the registry (fixes $0 cost on deepseek-auto entries)
- Move registry lock before FindModel call to fix data race

internal/providers/gemini.go

@@ -621,16 +621,10 @@ func (p *GeminiProvider) ChatCompletionStream(ctx context.Context, req *models.U
 		return nil, fmt.Errorf("Gemini API error (%d): %s", resp.StatusCode(), msg)
 	}
 
-	ch := make(chan *models.ChatCompletionStreamResponse)
-
-	go func() {
-		defer close(ch)
-		err := StreamGemini(resp.RawBody(), ch, req.Model)
-		if err != nil {
-			fmt.Printf("Gemini Stream error: %v\n", err)
-		}
-	}()
-
+	ch, err := StreamGemini(resp.RawBody(), req.Model)
+	if err != nil {
+		return nil, fmt.Errorf("gemini stream init error: %w", err)
+	}
 	return ch, nil
 }