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).

Eclair 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 fee and the two anchor outputs. Eclair 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

test("recv OpenChannel (funder cannot afford commitment fee)") { f =>
  import f._
  val open = alice2bob.expectMsgType[OpenChannel]
  // 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.
  // The funder owes the commit tx fee plus both anchors and pushes one sat
  // too much, leaving them with insufficient funds to meet those obligations.
  val spec = CommitmentSpec(Set.empty, open.feeratePerKw, 0 msat, 0 msat)
  val fees = commitTxTotalCost(open.dustLimitSatoshis, spec, ZeroFeeHtlcTxAnchorOutputsCommitmentFormat)
  bob ! open.copy(pushMsat = (open.fundingSatoshis - fees + 1.sat).toMilliSatoshi)
  val error = bob2alice.expectMsgType[Error]
  assert(error == Error(open.temporaryChannelId, CannotAffordFirstCommitFees(open.temporaryChannelId, missing = 1.sat, fees = fees).getMessage))
  listener.expectMsgType[ChannelAborted]
  awaitCond(bob.stateName == CLOSED)
}

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

ERROR f.a.e.c.fsm.Channel - can't pay the fee in first commitment: missing=12214 sat fees=24423 sat
                            while processing msg=FundingCreated in state=WAIT_FOR_FUNDING_CREATED

Suggested fix

Apply the same check that already exists in makeCommitTxs in validateParamsSingleFundedFundee.

Discovery

Found while fuzzing the v1 funding protocol with smite.