fix: stop row isolation after ambiguous send

tests/logger/queue_test.go

@@ -38,12 +38,15 @@ func (f *fakeBatch) Send() error  { return f.script.sendErr }
 func (f *fakeBatch) Abort() error { f.abortCalls++; return nil }
 
 type fakePool struct {
-	calls   int
-	queries []string
-	batches []*fakeBatch
-	scripts []batchScript
+	calls     int
+	queries   []string
+	batches   []*fakeBatch
+	scripts   []batchScript
+	unhealthy bool
 }
 
+func (p *fakePool) MarkUnhealthy() { p.unhealthy = true }
+
 type queueBackend struct {
 	prepare func(context.Context, string) (logger.Batch, error)
 	healthy atomic.Bool
@@ -215,6 +218,26 @@ func TestFlushAppendRecoveryCountsAmbiguousSingleSend(t *testing.T) {
 	}
 }
 
+func TestFlushAppendRecoveryStopsAfterFirstAmbiguousSend(t *testing.T) {
+	p := &fakePool{scripts: []batchScript{
+		{appendAt: 1, appendErr: errors.New("bad batch")},
+		{sendErr: errors.New("ack lost")},
+		{},
+	}}
+	entries := []*logger.LogEntry{{RequestID: "a"}, {RequestID: "b"}, {RequestID: "c"}}
+
+	result := logger.Flush(context.Background(), p, entries)
+	if result.Failed != 3 || result.Ambiguous != 1 || len(result.Retry) != 0 {
+		t.Fatalf("unexpected result: %+v", result)
+	}
+	if p.calls != 2 {
+		t.Fatalf("PrepareBatch calls=%d want 2; entries after ambiguous Send must not be attempted", p.calls)
+	}
+	if !p.unhealthy {
+		t.Fatal("ambiguous singleton Send did not immediately mark backend unhealthy")
+	}
+}
+
 func TestEstimatedBytesCoversStringsAndByteSlices(t *testing.T) {
 	entry := &logger.LogEntry{
 		RequestID: "1", Method: "22", Path: "333", Query: "4444", ClientIP: "55555", Error: "666666",