fix(proxy): filtra chunks SSE sin id y corta en [DONE]

Grok Build siempre usa streaming. El upstream (OpenCode Zen)
envía chunks no-estándar (inference-cost sin id, cost tras
[DONE]) que rompen el deserializador serde de Rust con
'missing field id'. streamResponse ahora filtra eventos SSE
que no tengan campo id y deja de forwardear después de [DONE].

internal/proxy/proxy.go

@@ -197,7 +197,8 @@ func (p *Proxy) ForwardChatCompletion(w http.ResponseWriter, r *http.Request, bo
 	return p.nonStreamResponse(w, resp, req.Model)
 }
 
-// streamResponse forwards SSE chunks directly from upstream to client.
+// streamResponse forwards SSE chunks from upstream to client, filtering out
+// non-standard events (missing `id`) and stopping after [DONE].
 func (p *Proxy) streamResponse(w http.ResponseWriter, resp *http.Response) error {
 	w.Header().Set("Content-Type", "text/event-stream")
 	w.Header().Set("Cache-Control", "no-cache")
@@ -212,17 +213,40 @@ func (p *Proxy) streamResponse(w http.ResponseWriter, resp *http.Response) error
 	scanner := bufio.NewScanner(resp.Body)
 	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)
 
+	done := false
 	for scanner.Scan() {
 		line := scanner.Text()
-		// Write the line. If the client disconnects, stop silently.
+		if done {
+			continue
+		}
+
+		if strings.HasPrefix(line, "data: ") {
+			data := strings.TrimPrefix(line, "data: ")
+			if data == "[DONE]" {
+				if _, err := fmt.Fprint(w, line+"\n"); err != nil {
+					return nil
+				}
+				flusher.Flush()
+				done = true
+				continue
+			}
+
+			// Only forward SSE events that have an "id" field — filters out
+			// non-standard chunks (e.g. x-opencode-type, standalone cost)
+			var obj map[string]any
+			if json.Unmarshal([]byte(data), &obj) == nil {
+				if _, ok := obj["id"]; !ok {
+					continue
+				}
+			}
+		}
+
 		if _, err := fmt.Fprint(w, line+"\n"); err != nil {
 			return nil
 		}
 		flusher.Flush()
 	}
 
-	// Scanner errors (e.g., connection reset) are not real errors
-	// after streaming has started — the client likely disconnected.
 	return nil
 }