test(xrp): skip the THORChain memo test, without weakening its assertion

XRP memo is not a supported feature yet, and the assertion is correct -- it
describes what the product needs, not what the code does. So it is skipped with
a specific reason rather than rewritten to accept the loss.

The memo cannot traverse hdwallet -> RippleSignTx. The KeepKey protobuf has no
memo field (RippleSignTx carries fields 1-6; RipplePayment carries
amount/destination/destination_tag), and hdwallet's rippleSignTx never reads
tx.value.memo -- it references msg.tx.value exactly twice, .fee and .msg. The
firmware therefore never receives a memo and cannot serialize one. No firmware
change makes this pass.

Making it green by asserting the memo is ABSENT would encode the bug as the
contract, so the body is untouched and the skip reason says so explicitly.

The host-side gap is tracked as keepkey/keepkey-vault#422: buildXrpTx routes
THORChain memos to tx.value.memo, index.ts signs via rippleSignTx, and the
memo is discarded -- producing a valid XRP payment to the THORChain inbound
vault with no routing memo. XRP swaps do not currently work, so this is armed
rather than actively losing funds, and the recommended host fix is to fail
closed in buildXrpTx rather than sign a memo-less payment.

Re-enable only when the signed serialization preserves the memo.

Author: BitHighlander

tests/test_msg_ripple_sign_tx.py

@@ -100,6 +100,20 @@ def test_sign(self):
         )
 
 
+    @unittest.skip(
+        "XRP memo is not a supported feature yet. A THORChain memo cannot "
+        "traverse hdwallet -> RippleSignTx: the protobuf has no memo field "
+        "(RippleSignTx carries 1-6, RipplePayment carries "
+        "amount/destination/destination_tag), and hdwallet's rippleSignTx "
+        "never reads tx.value.memo. The firmware therefore never receives it "
+        "and cannot serialize it. Tracked as keepkey/keepkey-vault#422.\n"
+        "\n"
+        "This assertion is CORRECT and is deliberately left intact: it "
+        "describes the behaviour the product needs. Do NOT make it pass by "
+        "asserting the memo is absent -- that would encode the bug as the "
+        "contract. Re-enable only when the signed serialization actually "
+        "preserves the memo."
+    )
     def test_sign_with_thorchain_memo(self):
         self.requires_fullFeature()
         self.requires_firmware("7.14.2")