fix(provider): fail the candidate on error-only streams instead of serving empty replies

Some upstreams (zen free pool) answer HTTP 200 with a single-chunk stream
whose only payload is finish_reason:"network_error" and an empty delta.
ChatStream used to hand that channel up as a success, so clients received
a laundered empty reply (agents ended their turn mid-loop) and the broken
slot kept its scheduling preference. ChatStream now holds back the first
chunk: an error-only done chunk fails the candidate before any byte
reaches the gateway, letting the scheduler fall through to the next
source. Standard finish reasons (including instant-empty "stop") are never
classified as errors.

internal/provider/provider_test.go

@@ -417,7 +417,8 @@ func TestChatClientCancelNotRecorded(t *testing.T) {
 }
 
 // TestChatStreamEmptyBodyNotSuccess: a 200 that yields zero chunks and no
-// [DONE] is a failure before the first chunk — the slot must back off.
+// [DONE] must fail the candidate (error return, no delivered chunks) so the
+// scheduler falls through to the next source, and record the failure.
 func TestChatStreamEmptyBodyNotSuccess(t *testing.T) {
 	up := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "text/event-stream")
@@ -429,15 +430,12 @@ func TestChatStreamEmptyBodyNotSuccess(t *testing.T) {
 		Model:    "m",
 		Messages: []types.ChatMessage{{Role: "user", Content: types.StringContent("x")}},
 	})
-	if err != nil {
-		t.Fatalf("stream: %v", err)
-	}
-	got := 0
-	for range ch {
-		got++
-	}
-	if got != 0 {
-		t.Fatalf("want empty stream, got %d chunks", got)
+	if err == nil {
+		if ch != nil {
+			for range ch {
+			}
+		}
+		t.Fatal("empty stream must fail the candidate for scheduler fallback")
 	}
 	eventually(t, 2*time.Second, func() bool {
 		_, fc, _ := p.ModelHealthInfo("m")
@@ -445,6 +443,62 @@ func TestChatStreamEmptyBodyNotSuccess(t *testing.T) {
 	}, "empty stream must record a failure")
 }
 
+// TestChatStreamErrorFinishFailsCandidate: zen free pool answers HTTP 200
+// with a single-chunk stream whose only payload is finish_reason:"network_error"
+// and an empty delta. That must fail the candidate (not deliver a laundered
+// empty reply), so the scheduler can fall through to the next source.
+func TestChatStreamErrorFinishFailsCandidate(t *testing.T) {
+	up := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		fmt.Fprint(w, `data: {"choices":[{"index":0,"finish_reason":"network_error","delta":{"role":"assistant","content":""}}]}`+"\n\n")
+		fmt.Fprint(w, "data: [DONE]\n\n")
+	}))
+	defer up.Close()
+	p := newTestProvider(t, src("mock", up.URL, "opencode", "m"))
+	_, err := p.ChatStream(context.Background(), &types.ChatRequest{
+		Model:    "m",
+		Messages: []types.ChatMessage{{Role: "user", Content: types.StringContent("x")}},
+	})
+	if err == nil {
+		t.Fatal("network_error-only stream must fail the candidate")
+	}
+	eventually(t, 2*time.Second, func() bool {
+		_, fc, _ := p.ModelHealthInfo("m")
+		return fc >= 1
+	}, "error-only stream must record a failure")
+}
+
+// TestChatStreamInstantEmptyStopStillDelivered: a legitimate completion that
+// ends immediately with the standard finish_reason:"stop" and zero content is
+// NOT an upstream error and must still reach the client.
+func TestChatStreamInstantEmptyStopStillDelivered(t *testing.T) {
+	up := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		fmt.Fprint(w, `data: {"choices":[{"index":0,"finish_reason":"stop","delta":{}}]}`+"\n\n")
+		fmt.Fprint(w, "data: [DONE]\n\n")
+	}))
+	defer up.Close()
+	p := newTestProvider(t, src("mock", up.URL, "openai", "m"))
+	ch, err := p.ChatStream(context.Background(), &types.ChatRequest{
+		Model:    "m",
+		Messages: []types.ChatMessage{{Role: "user", Content: types.StringContent("x")}},
+	})
+	if err != nil {
+		t.Fatalf("standard stop stream must be delivered: %v", err)
+	}
+	n := 0
+	for range ch {
+		n++
+	}
+	if n == 0 {
+		t.Fatal("expected at least the terminating chunk")
+	}
+	eventually(t, 2*time.Second, func() bool {
+		_, fc, _ := p.ModelHealthInfo("m")
+		return fc == 0
+	}, "standard stop must count as success")
+}
+
 // TestImageAutoUsesImageModel: AUTO image generation on a mixed source must
 // send the image-kind model id, never the best chat model.
 func TestImageAutoUsesImageModel(t *testing.T) {