fix: clamp negative amounts to 0 in AmountWrapper deserializer

The staking service can send negative remainingStakingAmount (e.g.
"-2282880") when the unstake amount exceeds the current balance.
The u64 deserializer rejected these with "invalid digit found in
string", preventing the intent from being processed at all.

The intent builder already guards with > 0 checks before using
amount values (e.g. partial unstake in build.rs), so clamping
negatives to 0 at deserialization is safe and lets the full unstake
path proceed correctly.

Ticket: BTC-3184

Author: lcovar

packages/wasm-solana/src/intent/types.rs

@@ -139,7 +139,13 @@ pub struct AmountWrapper {
     pub symbol: Option<String>,
 }
 
-/// Deserialize amount from either string or number (for JS BigInt compatibility)
+/// Deserialize amount from either string or number (for JS BigInt compatibility).
+///
+/// Negative values are clamped to 0 rather than rejected. The staking service can
+/// send negative `remainingStakingAmount` when the unstake amount exceeds the current
+/// balance. Callers already guard with `> 0` checks (e.g. build.rs partial unstake),
+/// so clamping is safe and avoids a deserialization error that would prevent the
+/// intent from being processed at all.
 fn deserialize_amount<'de, D>(deserializer: D) -> Result<u64, D::Error>
 where
     D: serde::Deserializer<'de>,
@@ -166,14 +172,19 @@ where
         where
             E: de::Error,
         {
-            u64::try_from(v).map_err(|_| de::Error::custom("negative amount"))
+            Ok(u64::try_from(v).unwrap_or(0))
         }
 
         fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
         where
             E: de::Error,
         {
-            v.parse().map_err(de::Error::custom)
+            // Try u64 first, fall back to i64 parse and clamp negatives to 0
+            v.parse::<u64>().or_else(|_| {
+                v.parse::<i64>()
+                    .map(|n| u64::try_from(n).unwrap_or(0))
+                    .map_err(de::Error::custom)
+            })
         }
     }