chore: add (previously) failing test case for u128 decomposition

- the code fix was already correct, but we add another edge case because we
have specialized decomposition code for u128 that we need to update and we
need a previously failing case, this serves as reference for the fix of the
specialized implementation

tfhe/src/core_crypto/commons/math/decomposition/tests.rs

@@ -242,6 +242,27 @@ fn test_decomposition_edge_case_sign_handling() {
     }
 }
 
+#[test]
+fn test_decomposition_edge_case_sign_handling_u128() {
+    let decomposer = SignedDecomposer::new(DecompositionBaseLog(40), DecompositionLevelCount(3));
+    // This value triggers a negative state at the start of the decomposition, invalid code using
+    // logic shift will wrongly compute an intermediate value by not keeping the sign of the
+    // state on the last level if base_log * (level_count + 1) > Scalar::BITS, the logic shift will
+    // shift in 0s instead of the 1s to keep the sign information
+    let val: u128 = 170141183460604905165246226680529368983;
+
+    let rounded = decomposer.closest_representable(val);
+    let recomp = decomposer.recompose(decomposer.decompose(val)).unwrap();
+    let decomp = decomposer.decompose(val);
+    assert_eq!(rounded, recomp);
+
+    let expected = [-421613125320i128, 482008863255, -549755813888];
+
+    for (term, expect) in decomp.zip(expected) {
+        assert_eq!(term.value() as i128, expect, "Problem with term {term:?}");
+    }
+}
+
 #[test]
 fn test_recompose_exhaustive() {
     let base_log = DecompositionBaseLog(10);