feat: Add plain-text editor for non-rich content channels (#13058)

# Pull Request Template

## Description

This PR restores the plain text editor for all channels except Website,
Email, and API.

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/dashboard/components-next/NewConversation/components/ComposeNewConversationForm.vue

@@ -200,6 +200,7 @@ const setSelectedContact = async ({ value, action, ...rest }) => {
 
 const handleInboxAction = ({ value, action, ...rest }) => {
   v$.value.$reset();
+  state.message = '';
   emit('updateTargetInbox', { ...rest });
   showInboxesDropdown.value = false;
   state.attachedFiles = [];
@@ -220,6 +221,7 @@ const removeSignatureFromMessage = () => {
 const removeTargetInbox = value => {
   v$.value.$reset();
   removeSignatureFromMessage();
+  state.message = '';
   emit('updateTargetInbox', value);
   state.attachedFiles = [];
 };
@@ -227,6 +229,7 @@ const removeTargetInbox = value => {
 const clearSelectedContact = () => {
   removeSignatureFromMessage();
   emit('clearSelectedContact');
+  state.message = '';
   state.attachedFiles = [];
 };