Block editor: Notify on block paste, matching copy and cut

[dhruvikpatel18]

What?

Closes #63964

Pasting whole blocks now shows a snackbar notice confirming the action, Pasted "Paragraph". for a single block, Pasted 3 blocks. for several matching the notice already shown on copy and cut. Applies to both the canvas and the List View.

Why?

Issue reports that duplicating a grid item in manual mode places the copy on top of the existing block, so there's no way to tell whether the action worked. Looking into it surfaced a broader, related gap: pasting blocks anywhere in the editor gives no confirmation at all, even though copy and cut both do.

The inconsistency exists even within paste itself:

Action	Notice today
Copy block	✅ Copied "Paragraph" to clipboard.
Cut block	✅ Moved "Paragraph" to clipboard.
Copy styles	✅ Styles copied to clipboard.
Paste styles	✅ Pasted styles to Paragraph.
Paste block	❌ nothing

Testing Instructions

1. Create a new post and add a Paragraph block with some text.
2. Select the whole block (press Esc from inside it, or click its edge) and copy it.
3. Paste. → A snackbar reads Pasted "Paragraph".
4. Select two or more blocks, copy, then paste. → A snackbar reads Pasted 2 blocks.
5. Open the List View, focus a row, copy it, focus another row, and paste. → The same notice appears.

Screenshots or screencast

Use of AI Tools

The Claude Code investigated the existing copy/cut notice paths, identified the four insertion sites, wrote useNotifyPaste, the handler call sites, the changelog entry, and the two e2e tests, and ran the verification below.

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Co-authored-by: dhruvikpatel18 <dhruvik18@git.wordpress.org>
Co-authored-by: noisysocks <noisysocks@git.wordpress.org>

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