fix(workflow): disable view switch during preview run instead of mounted guard

Simpler approach: disable the view picker toggle when preview is running,
preventing users from switching views during active runs.

This replaces the previous mounted ref guard approach (commits a0188bd9b5,
b7f1eb9b7b, 8332f0de2b) which added complexity to handle post-unmount
operations. Disabling the toggle is more direct and follows KISS principle.

Changes:
- Add disabled prop to ViewPicker based on isResponding state
- Revert mounted ref guards in use-chat-flow-control.ts
- Revert isMountedRef parameter in use-nodes/edges-interactions-without-sync.ts
- Revert defensive type check in markdown-utils.ts (no longer needed)

web/app/components/base/markdown/markdown-utils.ts

@@ -8,7 +8,7 @@ import { ALLOW_UNSAFE_DATA_SCHEME } from '@/config'
 
 export const preprocessLaTeX = (content: string) => {
   if (typeof content !== 'string')
-    return ''
+    return content
 
   const codeBlockRegex = /```[\s\S]*?```/g
   const codeBlocks = content.match(codeBlockRegex) || []
@@ -32,9 +32,6 @@ export const preprocessLaTeX = (content: string) => {
 }
 
 export const preprocessThinkTag = (content: string) => {
-  if (typeof content !== 'string')
-    return ''
-
   const thinkOpenTagRegex = /(<think>\s*)+/g
   const thinkCloseTagRegex = /(\s*<\/think>)+/g
   return flow([