### Steps to Reproduce:
1. Create a sign request and have 3 total signers (User, Customer, Employee)
2. Enable Signing Order and make the order as follows: (1) User, (2) Customer, (3) Employee
But make the User and Employee the same contact
3. Send and sign the request > Notice that (1) is able to sign for (3) immediately after, (2) has not signed yet.

### Description of the issue/feature this PR addresses: 
**Issue:** The signing order is ignored when the same user has to sign multiple times on a document, even if it is configured for a different person to sign in between. This happens because all signature request items are initialized in the 'sent' state upon creation, rather than strictly advancing based on the order. As a result, the system prematurely allows users to sign out of order and prompts them with their next turn too early.

**Solution:** To resolve this,  the controller was updated to include an `is_mail_sent = True` domain filter. This ensures that the UI's post-sign popup only displays documents where it is explicitly the user's active turn, rather than prompting a premature sign.

### Current behavior before PR: 
Users are able to sign prematurely, and the system will disregard the configured signing order.

### Desired behavior after PR: 
Users will only be prompted and able to sign a document when it is explicitly their turn, per the `mail_sent_order`. This way, documents are signed in order.

opw-6417327