Docs editor: tab indentation not announced to assistive technologies

What’s broken?

When the user presses the Tab key in the content area, the text is visually indented but no announcement is made by screen readers. The formatting change is not reported.

Impact

A blind user navigating with a screen reader presses Tab in the content area: no feedback is vocalized. They don't know if the indentation has been applied or at what level it is located.

What did you expect to happen?

When indentation is applied via the Tab key, a message must be conveyed by assistive technologies to inform the user of the indentation level change (e.g., "Indented", "Indentation level: 2").

Steps to reproduce

Open a Docs document
Place the cursor in the content area
Press the Tab key
Observe that the content is visually indented but no announcement is vocalized
RGAA criteria

Start by locating the Docs editor's Tab-key indentation handling and its accessibility announcement path. Reproduce the issue with a screen reader; it is done when applying indentation announces the formatting change and indentation level to assistive technologies.