i18n(ai-chat): rename typing indicator to "Thinking…"

Replace the AI chat typing indicator text "AI is typing…" with
"Thinking…".

- typing-indicator.tsx: use t("Thinking…") instead of t("AI is typing…")
- en-US: drop the now-redundant "AI is typing…" key (the "Thinking…"
  key already existed and was unused)
- ru-RU: rename the key to "Thinking…" with value "Думаю…"
- update related comments in message-list.tsx and the test file

apps/client/public/locales/en-US/translation.json

@@ -1147,7 +1147,6 @@
   "Take a look at the current document": "Take a look at the current document",
   "AI agent is typing…": "AI agent is typing…",
   "{{name}} is typing…": "{{name}} is typing…",
-  "AI is typing…": "AI is typing…",
   "Send": "Send",
   "Stop": "Stop",
   "Chat menu": "Chat menu",

apps/client/public/locales/ru-RU/translation.json

@@ -672,7 +672,7 @@
   "Take a look at the current document": "Посмотри текущий документ",
   "AI agent is typing…": "AI-агент печатает…",
   "{{name}} is typing…": "{{name}} печатает…",
-  "AI is typing…": "AI печатает…",
+  "Thinking…": "Думаю…",
   "Agent role": "Роль агента",
   "AI chat": "AI-чат",
   "AI chat is disabled for this workspace.": "AI-чат отключён для этого рабочего пространства.",