Starter page templates: correctly insert the pattern to the Content block

[fushar]

Context: p1733852360016879-slack-CBTN58FTJ

Proposed changes:

In Gutenberg 19.8, we started defaulting to template-locked rendering mode when creating a new page with this PR:

• Feature: Set editor rendering mode by post type WordPress/gutenberg#62304

This breaks the Starter Page Templates feature in wpcom at this line:

jetpack/projects/packages/jetpack-mu-wpcom/src/features/starter-page-templates/page-patterns-plugin.tsx

Line 97 in 3c012a8

replaceInnerBlocks( postContentBlock ? postContentBlock.clientId : '', blocks, false );

because in template-locked mode, the blocks includes the template parts, so the post content block might be buried in the middle inside some Group blocks.

This PR fixes that by recursively finding the Content block.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

1. Prepare an Atomic site.
2. Verify it's running Gutenberg 19.7.
3. Patch this PR to Jetpack Beta Tester's WordPress.com Features.
4. Activate TT5 theme.
5. Go to Pages -> Add New Page.
6. Verify you see the dotcom's Add a page modal.
7. Select any pattern.
8. Verify you can edit the pattern in the Editor afterwards.
9. Now, download and install and activate Gutenberg 19.8 https://github.com/WordPress/gutenberg/releases/tag/v19.8.0
10. Repeat the above steps and verify it still works.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the fix-starter-page-templates branch.

  • For jetpack-mu-wpcom changes, also add define( 'JETPACK_MU_WPCOM_LOAD_VIA_BETA_PLUGIN', true ); to your wp-config.php file.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix-starter-page-templates
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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• 🔴 Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
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[mmtr]

Lovely! Nice work @fushar