ui: Refactor data-attrs constants, enum for bool strings (#27002)

* refactor: Data-attribute constants + boolean string enum

* refactor: Use CSS class string constants

* refactor: Address review comments

tools/ui/tests/client/chat-form-enter-code-block.svelte.test.ts

@@ -2,7 +2,7 @@
 // fenced-code-block flow: while the caret sits inside a fenced
 // block region - closed, or still OPEN while the user is typing
 // one - plain Enter adds a line instead of submitting the message.
-// The textarea path is covered here end-to-end (the contenteditable
+// The textarea path is covered here end-to-end (the ChatFormInputRich
 // consumes the same case locally; see chat-form-input-rich).
 
 import ChatFormTestWrapper from './components/ChatFormTestWrapper.svelte';