Enhance script handling and temp ID resolution in ServeSession and Reconciler

- Improved error handling in ServeSession by rolling back temp IDs if encoding fails for new scripts.
- Added logic in applyPatch to remap echoed additions from the DescendantAdded handler, preventing duplicate instances.
- Updated API to resolve intermediate container temp IDs to real tree references, ensuring proper parent-child relationships for new scripts.
- Introduced new tests to verify the presence of server info flags and the resolution of temp IDs in scripts-only mode.

These changes aim to optimize script management and enhance the overall functionality of the session handling.

plugin/src/Reconciler/applyPatch.lua

@@ -67,6 +67,33 @@ local function applyPatch(instanceMap, patch)
 		end
 	end
 
+	-- Detect echoed additions from DescendantAdded handler.
+	-- When the plugin pre-inserts instances with temp IDs and the server
+	-- broadcasts the same instances with real Refs, remap the InstanceMap
+	-- entries rather than creating duplicates via reify.
+	local remapProgress = true
+	while remapProgress do
+		remapProgress = false
+		for id, virtualInstance in pairs(patch.added) do
+			local parentInstance = instanceMap.fromIds[virtualInstance.Parent]
+			if not parentInstance then continue end
+			for _, child in parentInstance:GetChildren() do
+				if child.Name == virtualInstance.Name
+					and child.ClassName == virtualInstance.ClassName then
+					local existingId = instanceMap.fromInstances[child]
+					if existingId and existingId ~= id then
+						Log.trace("Echo remap: {} -> {} for {}", existingId, id, child:GetFullName())
+						instanceMap:removeId(existingId)
+						instanceMap:insert(id, child)
+						patch.added[id] = nil
+						remapProgress = true
+						break
+					end
+				end
+			end
+		end
+	end
+
 	for id, virtualInstance in pairs(patch.added) do
 		if instanceMap.fromIds[id] ~= nil then
 			-- This instance already exists. We might've already added it in a

plugin/src/ServeSession.lua

@@ -961,7 +961,15 @@ function ServeSession:__connectScriptsOnlyWatchers()
 			end
 
 			local scriptTempId = string.gsub(HttpService:GenerateGUID(false), "-", ""):lower()
-			patch.added[scriptTempId] = encodeInstance(descendant, prevParentId)
+			local encoded = encodeInstance(descendant, prevParentId)
+			if not encoded then
+				Log.warn("Scripts-only: failed to encode new script {}, rolling back temp IDs", descendant:GetFullName())
+				for tempId in patch.added do
+					self.__instanceMap:removeId(tempId)
+				end
+				return
+			end
+			patch.added[scriptTempId] = encoded
 			self.__instanceMap:insert(scriptTempId, descendant)
 
 			Log.info(