Server-side session credential action handlers.

Dispatches on credential.payload.action to open, voucher, top_up, and close. Each handler updates per-channel deposit / voucher / spend balances through MPP.Session.Store.

open and topUp require funding verifiers, passed as the :verify_open / :verify_top_up options or the server-only "verify_open" / "verify_top_up" method-config keys. open calls verify.(payload, opts) and topUp calls verify.(payload, channel, opts). Without a verifier the action is rejected.

A verifier MUST confirm on-chain, at the finality the server requires, that the credential's transaction succeeded, that it targets the configured escrow_contract on the configured chain_id, and that it funds the credential's channelId with the configured payee and token. It returns the escrow's current channel state, or {:error, %MPP.Errors{}}:

{:ok, %{
  deposit: non_neg_integer(),      # total escrowed deposit
  settled: non_neg_integer(),      # paid out of the escrow, at most deposit
  close_requested: boolean(),      # a close is pending on-chain
  finalized: boolean(),            # the channel is closed on-chain
  payer: String.t(),               # open only, optional
  authorized_signer: String.t()    # open only, optional
}}

deposit, settled, close_requested, and finalized are all required; a result missing any of them, or of any other shape, is a verification failure. A zero deposit is rejected as an unfunded channel; close_requested or finalized reject the action as a closed channel. The payload's additionalDeposit and any configured or suggested deposit are never trusted as the ceiling.

Settled funds were already paid out of the escrow for earlier vouchers. At open the voucher's cumulativeAmount must lie between settled and deposit, the channel records settled as already spent, and only cumulativeAmount - settled counts toward the request. At topUp the deposit must increase, settled never decreases, and spent and cumulative are raised to it, so settled funds are never counted twice.

At open a verified payer must match the configured payer, and a verified signer becomes the channel signer (a descriptor must agree with it).

The voucher signer is fixed at open and stored on the channel. A zero signer address, from any source, means the payer signs. The signer is the descriptor's authorizedSigner once the descriptor hashes to channelId under the configured escrow_contract and chain_id; otherwise the verified signer, then the configured :authorized_signer, falling back to the payer. A descriptor's payee and token must match the configured recipient and token. A credential's top-level authorizedSigner must equal that signer. Vouchers and closes are verified against the stored signer only.