fix(store): backfill spans in run detail

internal/store/postgres/runs_test.go

@@ -128,3 +128,58 @@ func TestFindRunIndexForSpan_MatchesOpenRun(t *testing.T) {
 		t.Fatalf("expected span to attach to open run-2, got index %d", idx)
 	}
 }
+
+func TestAddSpanToRunSpans_MergesDuplicateSpanIDs(t *testing.T) {
+	rs := &runSpans{}
+
+	addSpanToRunSpans(rs, SpanRow{
+		SpanID: "span-1",
+		Name:   "tool.call",
+		Kind:   "tool",
+		Payload: json.RawMessage(`{
+			"payload":{"input":{"command":"ls"}}
+		}`),
+	})
+
+	addSpanToRunSpans(rs, SpanRow{
+		SpanID:  "span-1",
+		Status:  "error",
+		Duration: func() *int64 { v := int64(42); return &v }(),
+		Payload: json.RawMessage(`{
+			"payload":{"result_preview":"failed"}
+		}`),
+	})
+
+	if len(rs.order) != 1 {
+		t.Fatalf("expected one span order entry, got %d", len(rs.order))
+	}
+
+	span := rs.byID["span-1"]
+	if span == nil {
+		t.Fatal("expected merged span to be present")
+	}
+	if span.Name != "tool.call" {
+		t.Fatalf("expected name to be preserved, got %q", span.Name)
+	}
+	if span.Status != "error" {
+		t.Fatalf("expected error status to win, got %q", span.Status)
+	}
+	if span.Duration == nil || *span.Duration != 42 {
+		t.Fatalf("expected duration to be merged, got %#v", span.Duration)
+	}
+
+	var got map[string]any
+	if err := json.Unmarshal(span.Payload, &got); err != nil {
+		t.Fatalf("unmarshal merged payload: %v", err)
+	}
+	payload, ok := got["payload"].(map[string]any)
+	if !ok {
+		t.Fatal("expected merged payload object")
+	}
+	if _, ok := payload["input"].(map[string]any); !ok {
+		t.Fatal("expected input to remain after merge")
+	}
+	if payload["result_preview"] != "failed" {
+		t.Fatalf("expected result_preview to be merged, got %#v", payload["result_preview"])
+	}
+}