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

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 (e.g., $4 for a SELECT 1 query).

This ports the fix from mppx (wevm/mppx#196) and wallet
(tempoxyz/wallet#349):

1. The minimum close amount is now max(channel.spent, on_chain.settled)
   instead of highest_voucher_amount, allowing clients to close at
   actual usage cost.

2. The store update only overwrites highest_voucher_amount when the
   close amount is actually higher, preserving the original voucher
   for settlement if the close is at a lower (actual spent) amount.

Amp-Thread-ID: https://ampcode.com/threads/T-019d5503-2e85-70ff-81ed-b5093c920fd5
Co-authored-by: Amp <amp@ampcode.com>

Author: gakonst

src/protocol/methods/tempo/session_method.rs

@@ -821,12 +821,6 @@ where
             .parse()
             .map_err(|_| VerificationError::invalid_payload("invalid cumulativeAmount"))?;
 
-        if cumulative_amount < channel.highest_voucher_amount {
-            return Err(VerificationError::new(
-                "close voucher amount must be >= highest accepted voucher",
-            ));
-        }
-
         let channel_id_b256 = Self::parse_channel_id(channel_id_str)?;
         let escrow = self.resolve_escrow(details)?;
         let chain_id = self.resolve_chain_id(details);
@@ -839,9 +833,22 @@ where
                 "channel is finalized on-chain",
             ));
         }
-        if cumulative_amount < on_chain.settled {
+
+        // Allow closing at the actual spent amount rather than the highest
+        // pre-authorized voucher. This mirrors the fix in mppx (wevm/mppx#196)
+        // and wallet (tempoxyz/wallet#349) so that clients are only charged for
+        // actual usage, not the full reservation.
+        let min_close_amount = if channel.spent > on_chain.settled {
+            channel.spent
+        } else {
+            on_chain.settled
+        };
+        if cumulative_amount < min_close_amount {
             return Err(VerificationError::new(
-                "close voucher cumulativeAmount is below on-chain settled amount",
+                &format!(
+                    "close voucher amount must be >= {} (max of spent and on-chain settled)",
+                    min_close_amount,
+                ),
             ));
         }
         if cumulative_amount > on_chain.deposit {
@@ -948,10 +955,19 @@ where
                         Some(s) => s,
                         None => return Ok(None),
                     };
+                    let update_voucher = cumulative_amount > state.highest_voucher_amount;
                     Ok(Some(ChannelState {
                         deposit: on_chain.deposit,
-                        highest_voucher_amount: cumulative_amount,
-                        highest_voucher_signature: Some(sig_bytes),
+                        highest_voucher_amount: if update_voucher {
+                            cumulative_amount
+                        } else {
+                            state.highest_voucher_amount
+                        },
+                        highest_voucher_signature: if update_voucher {
+                            Some(sig_bytes)
+                        } else {
+                            state.highest_voucher_signature
+                        },
                         finalized: true,
                         ..state
                     }))