feat(chat): StreamerHub with ring buffer + cond + Done/Error/Cancel flows

internal/infra/llm/fake.go

@@ -15,6 +15,9 @@ type FakeClient struct {
 	StreamError error         // Stream() 立即返回的错误（如果非 nil）
 	TokenCount  int           // CountTokens 返回值
 	StreamCalls int
+	// BlockCh, if non-nil, causes Stream goroutine to block after emitting all
+	// Events until ctx is cancelled or BlockCh is closed. Useful for cancel tests.
+	BlockCh chan struct{}
 }
 
 // NewFake 构造一个 FakeClient，默认 provider="fake"。
@@ -33,6 +36,8 @@ func (f *FakeClient) Stream(ctx context.Context, modelID string, req Request) (<
 	events := append([]StreamEvent(nil), f.Events...)
 	f.mu.Unlock()
 
+	blockCh := f.BlockCh
+
 	ch := make(chan StreamEvent, len(events)+1)
 	go func() {
 		defer close(ch)
@@ -44,6 +49,14 @@ func (f *FakeClient) Stream(ctx context.Context, modelID string, req Request) (<
 			case ch <- e:
 			}
 		}
+		// If a block channel is set, pause here until ctx is cancelled or BlockCh
+		// is closed. This lets cancel tests confirm mid-stream cancellation.
+		if blockCh != nil {
+			select {
+			case <-ctx.Done():
+			case <-blockCh:
+			}
+		}
 	}()
 	return ch, nil
 }