xpay unable to pay bolt12 two hops away

[Lagrang3]

Formalizing an issue raised by @daywalker90 in #8972.

This tests always fails

def test_missing_blinded_htlcmax(node_factory):
    AMT_MSAT = 10000
    l1, l2, l3 = node_factory.get_nodes(3, opts={'may_reconnect': True})
    node_factory.join_nodes([l2, l3], wait_for_announce=True)
    offer = l2.rpc.offer('any')['bolt12']
    l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
    l1.rpc.connect(l3.info['id'], 'localhost', l3.port)
    b12 = l1.rpc.fetchinvoice(offer, AMT_MSAT)['invoice']

    b12_decode = l1.rpc.decode(b12)
    assert b12_decode["invoice_amount_msat"] == AMT_MSAT
    assert len(b12_decode["invoice_paths"]) == 1
    assert b12_decode["invoice_paths"][0]["first_node_id"] == l3.info["id"]
    assert len(b12_decode["invoice_paths"][0]["path"]) == 2

    node_factory.join_nodes([l1, l2], wait_for_announce=True)
    l1.rpc.xpay(invstring=b12, payer_note="Payment for a cup of coffee")

with error message:

{'code': 205, 'message': 'Failed: We could not find a usable set of paths.  The shortest path is 110x1x0->103x1x0->0x0x0, but 0x0x0/1 exceeds htlc_maximum_msat ~0msat'}

[Lagrang3]

Of course it has to fail!
l3 -> l2 doesn't have liquidity, l2 owns all of it.
The error seems not to reflect that, but it is factual: we just decode a bolt12 invoice saying that we have to use a blinded path but the htlc_max is not sufficient for the amount we want to send.