fix(web): guarantee unique assistant-ui thread message IDs (#706)

* fix(web): guarantee unique assistant-ui thread message IDs

Suffix duplicate kind:id pairs before ExternalStore sync; skip duplicate hub
rows in SessionChat normalization. Align outline scroll targets with the
new user-text thread id shape. Closes #704.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(web): reuse thread-id wrappers for useExternalMessageConverter cache

WeakMap stable BlockWithThreadMessageId objects keyed on reconciled block
refs so streaming appends do not invalidate assistant-ui converter caches
(PR #706 review).

Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

web/src/components/SessionChat.tsx

@@ -325,6 +325,9 @@ export function SessionChat(props: {
         const normalized: NormalizedMessage[] = []
         const seen = new Set<string>()
         for (const message of visibleMessages) {
+            if (seen.has(message.id)) {
+                continue
+            }
             seen.add(message.id)
             const cached = cache.get(message.id)
             if (cached && cached.source === message) {