Let keyboard users skip over RTE/MD formatting header

What does this MR do and why?

Add a button that only shows on focus at the start of the markdown and rich text formatting header, which when clicked will move focus to the input itself. This allows a keyboard user to move focus from outside of the editor to the actual text input in two keystrokes, instead of the 15+ necessary if focus is moved through each formatting button.

References

Screenshots or screen recordings

editor	Before	After
Markdown	Screen_Recording_2025-07-31_at_18.35.56	Screen_Recording_2025-07-31_at_18.31.08
Rich text	Screen_Recording_2025-07-31_at_18.35.03	Screen_Recording_2025-07-31_at_18.31.47

How to set up and validate locally

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Related to #343246 (closed), #513105 (closed)