plan renderer: render unknown nested blocks properly

[liamcervante]

This PR updates the rendering logic so that it properly renders the contents of a nested block that is becoming unknown. Previously, we'd only render a single item for the whole nested block. Now we should a single item being created that is unknown, and all existing entries being marked as deleted.

In addition, this change fixes a crash that would occur when nested blocks were becoming unknown and contained attributes that were forcing the resource to be replaced. We weren't properly accounting for the nested indices within the replacement list, and now since we are properly computing the diffs we are.

Fixes #35556

Target Release

v1.9.7

CHANGELOG entry

BUG FIXES

• Plan renderer: Fix crash that occurs when attempting to render unknown nested blocks that contain attributes forcing replacement.
• Plan renderer: Render complete changes within unknown nested blocks.

[kmoe]

needs backport label?

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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