[Building Sync-ups Tutorial] fixes in "Testing the sync-up detail" chapter (#3579)

* Update SyncUpDetail tutorial intro to clarify state communication

Modify tutorial introduction to accurately describe how the SyncUpDetail feature updates parent state using a shared property

* Update SyncUpDetail tutorial test code to use `modify` and `withLock`

Refactor test code to:
- Use `modify` for nested state updates
- Apply `withLock` for thread-safe state mutation when editing a sync-up

* Restore the SyncUpDetail tutorial for testing the delete flow with `@Shared` property.

* Remove the restored delete test

Since it is already tested in the navigation section, I finally remove it and update the tutorial text to inform of that with a link to it.

* Update Sources/ComposableArchitecture/Documentation.docc/Tutorials/BuildingSyncUps/06-SyncUpDetail/TestingSyncUpDetail.tutorial

---------

Co-authored-by: Stephen Celis <[email protected]>

Author: FredericRuaudel

Sources/ComposableArchitecture/Documentation.docc/Tutorials/BuildingSyncUps/06-SyncUpDetail/TestingSyncUpDetail-01-code-0005.swift

@@ -22,7 +22,7 @@ struct SyncUpDetailTests {
     var editedSyncUp = syncUp
     editedSyncUp.title = "Point-Free Evening Sync"
     await store.send(\.destination.edit.binding.syncUp, editedSyncUp) {
-      $0.destination?.edit?.syncUp = editedSyncUp
+      $0.destination?.modify(\.edit) { $0.syncUp = editedSyncUp }
     }
   }
 }

Sources/ComposableArchitecture/Documentation.docc/Tutorials/BuildingSyncUps/06-SyncUpDetail/TestingSyncUpDetail-01-code-0006.swift

@@ -22,12 +22,12 @@ struct SyncUpDetailTests {
     var editedSyncUp = syncUp
     editedSyncUp.title = "Point-Free Evening Sync"
     await store.send(\.destination.edit.binding.syncUp, editedSyncUp) {
-      $0.destination?.edit?.syncUp = editedSyncUp
+      $0.destination?.modify(\.edit) { $0.syncUp = editedSyncUp }
     }
 
     await store.send(.doneEditingButtonTapped) {
       $0.destination = nil
-      $0.syncUp = editedSyncUp
+      $0.$syncUp.withLock { $0 = editedSyncUp }
     }
   }
 }