fix rawHandling, pasteHandling for mixed content with blocks and classic

[saulyz]

What?

The rawHandler and the pasteHandler had a naive check for the block existence. The test was looking for string "<!-- /wp:" existence in the content. When mixed content exist, this would give positive result and would try to parse blocks thus wrongfully interpreting and failing.

Why?

When we convert legacy content to blocks sometimes whole article or some parts silently fail. Clicking "convert to blocks" does nothing - everything stays the same. In the Editor view one could not distinguish a problem content and would not get any error.

How?

Not changing the overall logic or the process, instead refining the check further checking if parsing has resulted with the same 'core/freeform' block. If content parsing results with the same and single freeform, let's continue the content conversion (previously process would stop).

Testing Instructions

Adding test files to PR that illustrate the issue.

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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[github-actions]

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[saulyz]

@ellatrix need your eyes here

[ellatrix]

I'm surprised this was never caught earlier.

[ellatrix]

I just cherry-picked this PR to the wp/6.6-beta-3 branch to get it included in the next release: e08c158