Prevent repeated paste from nesting actors as children

[Gopmyc]

Description

Fixes actor paste behavior that could unintentionally create a child hierarchy when pasting multiple times (Ctrl+V).

Paste now behaves as follows:

• if a target actor is selected, the new actor is pasted at the same level (same parent)
• if no target actor is selected, the actor is pasted at the root level

Also updates the PasteActor API comment to reflect this contract.

Related Issue(s)

Fixes #763

Review Guidance

What to verify:

• Copy an actor and press Ctrl+V multiple times: it should never create a child of the previously pasted copy
• The newly pasted actor should remain a sibling of the target actor (or be root if no target is selected)
• No other duplication/paste workflows should be affected

Screenshots/GIFs

N/A

AI Usage Disclosure

None

Checklist

• My code follows the project's code style guidelines
• When applicable, I have commented my code, particularly in hard-to-understand areas
• When applicable, I have updated the documentation accordingly
• My changes don't generate new warnings or errors
• I have reviewed and take responsibility for all code in this PR (including any AI-assisted contributions)