fix(cashu): preserve original case for custom CurrencyUnit values

Custom currency units were incorrectly being converted to uppercase
during deserialization. This change ensures custom units maintain
their original case while still allowing case-insensitive matching
for standard units.

CHANGELOG.md

@@ -23,6 +23,8 @@
 - cdk-mintd: Updated to use new mint lifecycle methods for improved service management ([thesimplekid]).
 - cdk-integration-tests: Updated test utilities to use new mint lifecycle management ([thesimplekid]).
 
+### Fixed
+- cashu: Fixed CurrencyUnit custom units preserving original case instead of being converted to uppercase ([thesimplekid]).
 
 
 ## [0.11.0](https://github.com/cashubtc/cdk/releases/tag/v0.11.0)

crates/cashu/src/nuts/nut00/mod.rs

@@ -591,14 +591,14 @@ impl CurrencyUnit {
 impl FromStr for CurrencyUnit {
     type Err = Error;
     fn from_str(value: &str) -> Result<Self, Self::Err> {
-        let value = &value.to_uppercase();
-        match value.as_str() {
+        let upper_value = value.to_uppercase();
+        match upper_value.as_str() {
             "SAT" => Ok(Self::Sat),
             "MSAT" => Ok(Self::Msat),
             "USD" => Ok(Self::Usd),
             "EUR" => Ok(Self::Eur),
             "AUTH" => Ok(Self::Auth),
-            c => Ok(Self::Custom(c.to_string())),
+            _ => Ok(Self::Custom(value.to_string())),
         }
     }
 }
@@ -977,4 +977,12 @@ mod tests {
             .unwrap();
         assert_eq!(b.len(), 1);
     }
+
+    #[test]
+    fn custom_unit_ser_der() {
+        let unit = CurrencyUnit::Custom(String::from("test"));
+        let serialized = serde_json::to_string(&unit).unwrap();
+        let deserialized: CurrencyUnit = serde_json::from_str(&serialized).unwrap();
+        assert_eq!(unit, deserialized)
+    }
 }