When the paragraph starts with Annotation with SkipPositionBefore = true, TabForward command does not detect that the caret is actually at the beginning of paragraph, and inserts Tab character instead of changing left indent.

Workaround: Subscribe to CommandExecuting and cancel the InsertTextCommand with parameter "\t" if the caret is at such position

Steps to reproduce:
- Insert a paragraph in the document.
- Put the caret at the beginning of the paragraph and insert bookmark (without selection).
- Put the caret at the beginning of the paragraph and press tab.
Expected: Paragraph's Left indent is changed.
Observed: Tab character is inserted.