fix(chat): 恢复 chat 输入框的 bubble menu 以支持清除格式 (MUL-5106) (#5727)

Chat 输入框是共用的 Tiptap 编辑器,`# ` input rule 与 Markdown/HTML
粘贴都会把行变成标题,但此前 chat 传了 showBubbleMenu={false},关掉了
唯一能把标题降回正文("Normal text" / setParagraph)及切换其它格式的
浮动工具栏,导致格式一旦产生就无法去掉。

组件本就暴露了 showBubbleMenu 配置能力,这里改为启用即可,和 Linear 的
交互一致;attach、@ 提及、图片/文件等内联能力不受影响。

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: multica-agent <github@multica.ai>

packages/views/chat/components/chat-input.tsx

@@ -537,9 +537,13 @@ export function ChatInput({
             mentionMode={contextItems ? "context" : "default"}
             mentionContextItems={contextItems}
             enableSlashCommands
-            // Chat is short-form — the floating formatting toolbar is
-            // more distraction than feature here.
-            showBubbleMenu={false}
+            // The bubble menu carries the only affordance that can strip
+            // formatting — "Normal text" (setParagraph) plus the mark/list
+            // toggles. Once a `# ` input rule or a Markdown/HTML paste turns a
+            // line into a heading, chat has no other way to remove it, so
+            // without the bubble menu formatting can be created but never
+            // undone (MUL-5106).
+            showBubbleMenu
           />
         </div>
         {(uploadEnabled || leftAdornment) && (