fix: cooperative close uses actual spent amount instead of voucher ceiling

[gakonst]

Problem

On cooperative close, the server required the close voucher's cumulativeAmount to be >= highest_voucher_amount (the pre-authorized reservation). This meant clients were charged the full reservation even when actual usage was much lower.

Reproduced with Dune MPP: A SELECT 1 query (88ms execution, 4 bytes result) was charged the full $4.00 reservation instead of actual compute cost.

Root Cause

handle_close() validated:

if cumulative_amount < channel.highest_voucher_amount { // ❌ rejects actual-spend closes

And unconditionally overwrote the voucher:

highest_voucher_amount: cumulative_amount, // ❌ always stomps

Fix

Ports the fix from mppx (wevm/mppx#196) and wallet (tempoxyz/wallet#349).

Now matches the mppx handleClose implementation exactly:
Session.ts#L837-L846

1. Spent check: cumulative_amount >= channel.spent (must cover actual usage)
2. Settled check: cumulative_amount > on_chain.settled (must exceed on-chain settled, strict)
3. Store update only overwrites highest_voucher_amount when the close amount is actually higher, preserving the original voucher for settlement.

Testing

All 240 existing tests pass. The behavioral change only affects cooperative close when the client sends a voucher below the highest pre-authorized amount but above actual spend — previously rejected, now accepted.

[github-actions]

⚠️ Changelog not found.

A changelog entry is required before merging. We've generated a suggested changelog based on your changes:

Preview

---
mpp: patch
---

Fixed close voucher validation to check against the actual spent amount (instead of `highest_voucher_amount`) and on-chain settled/deposit values via a dedicated `validate_close_amount` function. Also fixed on-chain channel state updates to only replace the highest voucher when the new amount is strictly greater. Added comprehensive unit tests mirroring the mppx reference implementation.

Add changelog to commit this to your branch.