Bug: Backspace does not move the caret to the previous shadow root element

Lexical version: 0.45 latest

## Steps To Reproduce

1. Insert a shadow root element, such as CollapsibleContainer in the playground
2. Insert a paragraph of text after the shadow root element
3. Insert your cursor in the middle of the paragraph text and press Backspace

Link to code example: https://playground.lexical.dev/#doc=H4sIAAAAAAAAE9WTT2_CMAzFv8s7Z6gdMFiOHDhP4jjtkDUGIoW4cl00hPrdp5T9K9MmbTtMnCLbes8_28oR5IOyrNQpwR4hzJrfahuiF0qw94PgwcAHoUoDJ9jUxmiwZtk5hQUMQvKUFLYw0ENNsKiduI24eguDPUnTC0sDpSddviiLU7jSQ8wSdOZrgv_A-XmbimN0dRMeI11p0EiDdt_P50ldiL3b-p1oxz4bp5yJMGhe6U6wsFgy4w2gzw16XsSuKk6ay38kP7d0IZGcTcF13rdKS5_O8dsLLJxc5AX6Xz9A7Qyia3Tl9uRhy9m8vJ3dXE_G8-nYoOFWqqy7i-6wEW6T_6hGMZpMRwW6ZxU_aJVeBAAA

## The current behavior

The caret gets stuck at the beginning of a paragraph and does not move to the previous element

https://github.com/user-attachments/assets/06250fb6-58a4-4843-81ab-fac36da77e82

## The expected behavior

The caret moves to the nearest available text-containing node, and the text after the caret is appended to it

## Impact of fix

Similar behavior works in Notion and seems entirely expected. The only caveat is what happens if the last element in the shadow root is a decorator, such as an embed video. In this case, I see several options:
- The caret stops at the paragraph and does not move anywhere.
- The caret always jumps over any decorators and the text appends to the nearest available node (like in Notion)

Reproduce the issue in the linked playground using a CollapsibleContainer shadow root, a following paragraph, and Backspace from the paragraph. Trace the caret and nearest text-containing-node behavior from that reproduction, including the decorator case described in the issue. Done means Backspace moves the caret to the nearest available text-containing node and appends the remaining text there.

Written by the indexing model from the issue text.