Inconsistent cleanup of OMath objects when using Ctrl+A, Del on documents starting with equations

## Bug Description
Selecting an entire document (using Ctrl+A) and then deleting the document's contents (pressing Del) when the document starts with a display OMath expression will randomly leave an empty OMath object at the start of the document.

Here is a video demonstrating the issue: [Video: Ctrl+A, Del does not consistently cleanup math zones (3 minutes)](https://iu.mediaspace.kaltura.com/media/t/1_pvpcpucq)

## Current Behavior
When a document begins with an OMath object, selecting all of the document's content (using Ctrl+A), and then deleting it (pressing Del) randomly leaves an empty OMath object at the start of the document.

## Expected Behavior
When a document begins with an OMath object, selecting all of the document's content (using Ctrl+A), and then deleting it (pressing Del) completely deletes the document's contents including all OMath objects.

## Steps to Reproduce
1. Open Microsoft Word 365 and begin a blank document
2. Enter the following mathematical expression using the Equation Editor using the UnicodeMath format: `a\cup b`
3. Convert the expression to Professional
4. Enter a sentence on a new line below the math expression
5. Press Ctrl+A to select the entire document
6. Press Del to delete all of the contents
7. Start typing some text an observe if the text is in the Normal style or if it's inside of the Equation Editor
8. Press Ctrl+A to select the entire document
9. Press Del
10. Go back to step 2 and repeat.
- Repeat as many times as needed to see that the behavior is inconsistent.
- Sometimes after pressing Ctrl+A, Del the document will begin in the Normal style and default font
- Sometimes after pressing Ctrl+A, Del the document will begin in a math zone (empty OMath object)

## Environment
- Microsoft Windows 11 (fresh build)
- Windows 11 Home Version 10.0.22631 Build 22631
- Microsoft Word 365 Version 2410 (Build 18129.20158 Click-to-Run) Current Channel
- Default ReadAloud settings
- Microsoft David voice
- Default reading speed

## Impact
Non-visual users will expect that pressing Ctrl+A, then Del will completely delete the document's contents. They may inadvertently type text at the start thinking they are in the Normal style, when they are actually in the Equation Editor.

No source file, test, or entry point is named in the issue. First reproduce the Ctrl+A, Del sequence in Microsoft Word 365 using the listed equation and environment, repeating it until the empty OMath object appears. Done means deleting the full document contents consistently leaves Normal-style text insertion with no empty OMath object.

Written by the indexing model from the issue text.