Failure to reject `open_channel` where funder cannot pay commitment fee

[morehouse]

BOLT 2 requires the receiver of open_channel to fail the channel when the funder's amount cannot cover the initial commitment fee:

The receiving node MUST fail the channel if:
  - the funder's amount for the initial commitment transaction is not sufficient for full [fee payment](03-transactions.md#fee-payment).

CLN implements the rule, but too late. The channel isn't rejected until later in the flow, after receiving funding_created.

Impact

A peer can set a push_msat for the channel that leaves insufficient funds to pay the initial commitment fees. CLN initially accepts such channels and responds with accept_channel, even though it will inevitably reject them one roundtrip later in the funding flow (after receiving funding_created). This is entirely a spec compliance issue.

Reproduction

@pytest.mark.openchannel('v1')
def test_open_channel_funder_cannot_afford_fee(node_factory, bitcoind):
    """A funder left short of the commitment fee must be rejected.

    BOLT 2: the receiving node MUST fail the channel if the funder's amount for
    the initial commitment transaction is not sufficient for full fee payment.
    CLN only notices in initial_commit_tx(), after accept_channel has gone out.
    """
    l1 = node_factory.get_node()

    chain_hash = bytes.fromhex(bitcoind.rpc.getblockhash(0))[::-1]
    # Use the node's own opening feerate, so we're inside its accepted range.
    feerate = l1.rpc.feerates('perkw')['perkw']['opening']
    funding_sat = 16777216

    # The funder pays the commitment fee out of its own balance, so pushing the
    # balance away leaves it with nothing to pay from.
    push_msat = funding_sat * 1000

    lconn, channel_type = raw_peer_connect(l1)
    temp_chan_id = os.urandom(32)
    send_open_channel(lconn, chain_hash, temp_chan_id, funding_sat,
                      push_msat, feerate, channel_type)

    mtype = read_channel_reply(lconn)
    assert mtype in (WIRE_WARNING, WIRE_ERROR), \
        "funder left with {} sat to pay the commitment fee was not rejected (got msgtype {})".format(
            funding_sat - push_msat // 1000, mtype)

If we continue to flow to funding_created, the initial_commit_tx check rejects the channel:

UNUSUAL 034f355b...-openingd-chan#1: Funder cannot afford fee on initial commitment transaction

Suggested fix

Apply the same check that already exists in initial_commit_tx immediately after receiving accept_channel.

Discovery

Found while fuzzing the v1 funding protocol with smite.