Make accessible optional in HDAObject

It is probably enough to serialize the `accessible` property only when requesting collection contents displayed in the UI.

client/src/api/schema/schema.ts

@@ -6084,7 +6084,7 @@ export interface components {
          */
         HDAObject: {
             /** Accessible */
-            accessible: boolean;
+            accessible?: boolean | null;
             /** Copied From Ldda Id */
             copied_from_ldda_id?: string | null;
             /**

lib/galaxy/schema/schema.py

@@ -939,7 +939,7 @@ class HDAObject(Model, WithModelClass):
     history_id: HistoryID
     tags: List[str]
     copied_from_ldda_id: Optional[EncodedDatabaseIdField] = None
-    accessible: bool
+    accessible: Optional[bool] = None
     model_config = ConfigDict(extra="allow")