IJPL-252099 fix duplicated prefix

(cherry picked from commit c98a44ce047376fea8db120a4d730f67c41bd4eb)

GitOrigin-RevId: e0e3da83422e24cb3239c67e989e8ca0d61b6c43

platform/completion/common/src/protocol/RpcLookupElementEvent.kt

@@ -42,14 +42,23 @@ sealed interface RpcLookupElementEvent {
 
   /**
    * the lookup is closed with completion
+   *
+   * [requestId] is the request the chosen item belongs to, and it is what the backend must resolve
+   * [selectedItemId] against: items live in a per-request storage
+   * (`BackendCompletionRequestSession.findCompletionResult`), and the mirror lookup's arranger can still belong to an
+   * older request when this event is handled (IJPL-252099 — the frontend swaps a stale-seeded request for a fresh one
+   * right before the accept). Resolving against the arranger's own session instead would fail to find the item and
+   * insert one carrying a shorter prefix matcher, leaving the leading typed characters in the document.
    */
   @Serializable
   data class ItemSelected(
     val projectId: ProjectId,
+    val requestId: RpcCompletionRequestId,
     val selectedItemId: RpcCompletionItemId? = null,
   ) : RpcLookupElementEvent {
     override fun toString(): String = buildToString("ItemSelected") {
       field("projectId", projectId)
+      field("requestId", requestId)
       fieldWithNullDefault("selectedItemId", selectedItemId)
     }
   }