Writing flow: respect text fields inside open shadow roots

[dilipom13]

What

Fixes #82755

Select-all and paste now use the composed event target, so text fields inside open shadow roots keep native Cmd/Ctrl+A and paste behaviour instead of triggering block-level operations.

How

• Add getEventTarget() helper using event.composedPath()[0]
• Use it in writing-flow select-all and clipboard handlers
• Let native clipboard behaviour win when the composed target is a text field

Test plan

• Reproduce with the console snippet from Editor keyboard/clipboard handlers misfire for editable elements inside shadow DOM #82755
• Focus the shadow textarea and press Cmd/Ctrl+A - text should select, not all blocks
• Paste into the shadow textarea - text should insert, block should not be replaced
• Confirm Custom HTML / normal text fields still behave as before

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: dilipom13 <dilip2615@git.wordpress.org>
Co-authored-by: sirreal <jonsurrell@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[dilipom13]

Could you please add the [Type] Bug label? I don't have permission to add labels on this repo.