[Dev UI] Model playground - enabling append-mode should trigger existing messages to become editable

[MichaelDoyle]

In append mode (i.e. chat mode) any message can be edited, deleted, etc. Initially when ticking append mode, model generated messages currently do not "Switch" over to editable.

Incorrectly showing the model message as "uneditable"

After clicking "add message", it the "remove button" becomes enabled, but the text input is still not editable.

Requirement: check append mode is ticked, refresh all messages to ensure they are editable.