Make the rewind happy test tolerant of the settle-release window

Why: run terminal status persists before the conversation claim is
released; on the slow CI runner the rewind POST landed inside that
window and got a 409. Retry the POST until the claim frees (bounded).
Evidence: go test -count=5 ./internal/server/ -run TestRewindFilesHTTPHappy
green; full suite green.
Impact: test-only.
Refs: Agent-System-Backend#94 #97

internal/server/rewind_http_test.go

@@ -86,7 +86,21 @@ func TestRewindFilesHTTPHappy(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	code, out := doJSON(t, h, "POST", "/api/conversations/"+convID+"/rewind", map[string]any{"messageId": userMsgID})
+	// run 行已终态与释放会话占用之间有个 settle 窗口（CI 慢机上会撞 409）：
+	// 有界重试直到放行。
+	var code int
+	var out map[string]any
+	deadline2 := time.Now().Add(3 * time.Second)
+	for {
+		code, out = doJSON(t, h, "POST", "/api/conversations/"+convID+"/rewind", map[string]any{"messageId": userMsgID})
+		if code != http.StatusConflict {
+			break
+		}
+		if time.Now().After(deadline2) {
+			break
+		}
+		time.Sleep(15 * time.Millisecond)
+	}
 	if code != http.StatusOK {
 		t.Fatalf("rewind status %d: %v", code, out)
 	}