fix(chat,testing): memoize welcome message and add selectDistinct to dbChainMock.db

Why:
- Welcome ChatMessage was rebuilt inline each render, producing a fresh
  timestamp and new array identity — cascading to ChatMessageContainer
  and VoiceInterface props on every tick.
- dbChainMockFns exports selectDistinct/selectDistinctOn but the
  dbChainMock.db object omitted them, so tests that stub those builders
  hit undefined on the mocked module.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: waleedlatif1

apps/sim/app/chat/[identifier]/chat.tsx

@@ -1,6 +1,6 @@
 'use client'
 
-import { type RefObject, useCallback, useEffect, useRef, useState } from 'react'
+import { type RefObject, useCallback, useEffect, useMemo, useRef, useState } from 'react'
 import { createLogger } from '@sim/logger'
 import { generateId } from '@sim/utils/id'
 import { noop } from '@/lib/core/utils/request'
@@ -109,17 +109,21 @@ export default function ChatClient({ identifier }: { identifier: string }) {
   const chatConfig = chatConfigResult?.kind === 'config' ? chatConfigResult.config : null
 
   const welcomeMessage = chatConfig?.customizations?.welcomeMessage
-  const displayMessages: ChatMessage[] = welcomeMessage
-    ? [
-        {
-          id: 'welcome',
-          content: welcomeMessage,
-          type: 'assistant',
-          timestamp: new Date(),
-          isInitialMessage: true,
-        },
-        ...messages,
-      ]
+  const welcomeChatMessage = useMemo<ChatMessage | null>(
+    () =>
+      welcomeMessage
+        ? {
+            id: 'welcome',
+            content: welcomeMessage,
+            type: 'assistant',
+            timestamp: new Date(),
+            isInitialMessage: true,
+          }
+        : null,
+    [welcomeMessage]
+  )
+  const displayMessages: ChatMessage[] = welcomeChatMessage
+    ? [welcomeChatMessage, ...messages]
     : messages
 
   const { isStreamingResponse, abortControllerRef, stopStreaming, handleStreamedResponse } =

packages/testing/src/mocks/database.mock.ts

@@ -189,6 +189,8 @@ export function resetDbChainMock(): void {
 export const dbChainMock = {
   db: {
     select,
+    selectDistinct,
+    selectDistinctOn,
     insert,
     update,
     delete: del,