disallow dragging collections to dataset-only inputs

This refers to `FormData` elements without the collection variant (only single or multiple dataset variants).

Explained in galaxyproject#19866 (comment)

Author: ahmedhamidawan

client/src/components/Form/Elements/FormData/FormData.vue

@@ -110,6 +110,13 @@ const keepOptionsUpdate = ref(0);
  */
 const canBrowse = computed(() => variant.value && !!variant.value.find((v) => v.src === SOURCE.DATASET));
 
+/**
+ * A list of valid `src`s for the variant
+ */
+const validSrcs = computed(() => {
+    return variant.value ? variant.value.map((v) => v.src) : [];
+});
+
 /**
  * Provides the currently shown source type
  */
@@ -572,7 +579,12 @@ function canAcceptSrc(historyContentType: "dataset" | "dataset_collection", coll
         }
     } else if (historyContentType === "dataset_collection") {
         if (props.type === "data") {
-            // collection can always be mapped over a data input ... in theory.
+            // if this input doesn't accept collections at all, return false
+            if (!validSrcs.value.includes(SOURCE.COLLECTION)) {
+                $emit("alert", "dataset collection is not a valid input for this dataset parameter.");
+                return false;
+            }
+            // otherwise, collection can always be mapped over a data input ... in theory.
             // One day we should also validate the map over model
             return true;
         }