Contact Form: unsaved changes are lost switching editor modes

#### Steps to reproduce the issue

1. Create a new post or page
2. Add a contact form in the visual editor
3. Edit the form and add a new field
4. Switch to the text mode in the editor

#### What I expected

A prompt stating there are unsaved changes

#### What happened instead

Unsaved changes are lost

#### Screenshot


Reproduce the loss by creating a post or page, adding a contact form in the visual editor, adding a field, and switching to text mode. Trace the contact-form editor integration and mode switch, then verify that switching modes prompts about unsaved changes and preserves the added field.

Written by the indexing model from the issue text.