Can't use ] character in contact form attributes.

#### Steps to reproduce the issue

1. In the New Post screen, click "Add Contact Form".
2. Set the `subject` attribute of the `[contact-form]` shortcode tag to `"[Contact Form Submission]"`.

#### What I expected

The form would display correctly on the website.

#### What happened instead

Everything after the `]` in the `subject` attribute is output on the webpage.

For example, this contact form shortcode:

```
[contact-form subject="[Contact Form Submission]" to="admin@example.com"][contact-field label="Name" type="name" required="true" /][contact-field label="Email" type="email" required="true" /][contact-field label="Website" type="url" /][contact-field label="Message" type="textarea" /][/contact-form]
```

inadvertently leaks my email address:

Start from the contact-form shortcode entry point and reproduce the issue using the subject attribute shown in the report. Add a regression test for a subject containing a literal `]`, then verify that the complete shortcode renders without outputting the remaining attributes or exposing the email address.

Written by the indexing model from the issue text.