💻 feat: Deeper MCP UI integration in the Chat UI (#9669)

* 💻 feat: deeper MCP UI integration in the chat UI using plugins

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Co-authored-by: Samuel Path <samuel.path@shopify.com>
Co-authored-by: Pierre-Luc Godin <pierreluc.godin@shopify.com>

* 💻 refactor: Migrate MCP UI resources from index-based to ID-based referencing

- Replace index-based resource markers with stable resource IDs
- Update plugin to parse \ui{resourceId} format instead of \ui0
- Refactor components to use useMessagesOperations instead of useSubmitMessage
- Add ShareMessagesProvider for UI resources in share view
- Add useConversationUIResources hook for cross-turn resource lookups
- Update parsers to generate resource IDs from content hashes
- Update all tests to use resource IDs instead of indices
- Add sandbox permissions for iframe popups
- Remove deprecated MCP tool context instructions

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Co-authored-by: Pierre-Luc Godin <pierreluc.godin@shopify.com>

client/src/Providers/MessagesViewContext.tsx

@@ -28,6 +28,10 @@ interface MessagesViewContextValue {
 
 const MessagesViewContext = createContext<MessagesViewContextValue | undefined>(undefined);
 
+// Export the context so it can be provided by other providers (e.g., ShareMessagesProvider)
+export { MessagesViewContext };
+export type { MessagesViewContextValue };
+
 export function MessagesViewProvider({ children }: { children: React.ReactNode }) {
   const chatContext = useChatContext();
   const addedChatContext = useAddedChatContext();