Fixed customized message for draft notice

[sabbir1991]

What?

Issue #77014

Updates the Draft saved notice so it can use a post type label, similar to the existing trash notice behavior.

Why?

The Draft saved message was hardcoded, which made it unavailable to post types that need a custom save notice.

How?

Adds a new post type label for draft saves and updates the editor success notice logic to use that label instead of the hardcoded string.

Testing Instructions

1. Register a custom post type with a custom draft-save label.
2. Open a new draft for that post type in the editor.
3. Save the draft.
4. Confirm the snackbar shows the custom draft-save message.
5. Save a regular post draft and confirm the default Draft saved message still appears.

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[Mamaduka]

Closing as this doesn't fix the issue. We can continue the discussion in the ticket.