fix: revert "prevent prematurely cloning light dom slot vnodes"

[nolanlawson]

Details

This reverts commit 1105823 (#4258).

The fix for #4446 is ready to go (#4452), but for 252 (Winter '25) I think it would be pragmatic to just revert #4258 entirely.

This will make the 252 behavior the same as 250. I think this is the right choice, because:

1. fix: fix light dom slot forwarding bug #4452 is quite complex, and we probably want time to bake in main
2. The bug introduced by fix: prevent prematurely cloning light dom slot vnodes #4258 is IMO more severe than the bug it was fixing (not rendering DOM nodes at all is more severe than not calling disconnectedCallback – synthetic lifecycle already has plenty of bugs where it doesn't call disconnectedCallback, which mostly manifests as memory leaks rather than DOM nodes failing to render).

Does this pull request introduce a breaking change?

• 😮‍💨 No, it does not introduce a breaking change.

Does this pull request introduce an observable change?

• 🔬 Yes, it does include an observable change.

Yes, 252 behavior will go back to 250 behavior (disconnectedCallback not firing correctly in certain tricky cases with light DOM slot forwarding).

[jmsjtu]

I agree with this approach, the number of impacted customers was very low prior to this fix too.

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