refactor: Use Pair for MsoMdocItem deduplication instead of string concatenation

Replaced string concatenation (namespace + elementIdentifier) used in distinctBy
with a Pair(namespace, elementIdentifier).

Using string concatenation as a uniqueness key is fragile and can theoretically
lead to collisions. A Pair provides structural equality and makes the intent of
deduplication explicit and type-safe.

Author: gstamatop

common-feature/src/main/java/eu/europa/ec/commonfeature/ui/request/transformer/RequestTransformer.kt

@@ -186,7 +186,7 @@ object RequestTransformer {
 
                 val disclosedItems = mDocItems
                     .distinctBy {
-                        it.namespace + it.elementIdentifier
+                        it.namespace to it.elementIdentifier
                     } + sdJwtItems
 
                 return@mapNotNull if (disclosedItems.isNotEmpty()) {