fix: render synced patterns inside Group blocks in newsletters

[jason10lee]

Changes proposed

Synced patterns (reusable blocks / core/block) placed inside Group blocks were silently stripped from newsletter email output. This happened because:

1. is_empty_block() treated core/block as empty (it has no innerHTML — content lives in a referenced wp_block post). This is similar to a pattern that prevented Remote Data Blocks from rendering on send.
2. render_mjml_component() had no handler for core/block — resolution only happened at the top level in post_to_mjml_components()

This PR extracts the reusable block resolution into a shared resolve_reusable_block() method used by both code paths, with additional hardening:

• Post validation — checks post_type === 'wp_block' and post_status === 'publish' (previously unchecked)
• Circular reference guard — static stack prevents infinite recursion if patterns reference each other
• Filter consistency — uses get_valid_post_blocks() so the newspack_newsletters_newsletter_content filter is applied to nested patterns, matching top-level behavior
• Stale ref handling — deleted/invalid refs return empty instead of rendering empty wrapper markup

Resolves NPPM-2610: Bug: Some Patterns don't display in sent Newsletters when placed inside of a "Group" block

How to test

1. Create a synced pattern containing a Heading and a Paragraph block
2. Create a newsletter, add a Group block, insert the synced pattern inside it
3. Preview the newsletter — the pattern content should appear in the email output
4. Also verify: a top-level synced pattern (not in a group) still renders correctly

[Copilot]

Pull request overview

Fixes newsletter MJML rendering so synced patterns (core/block / reusable blocks) render correctly when nested inside core/group blocks, adding safer reusable-block resolution behavior.

Changes:

• Add shared reusable-block resolution with wp_block validation and circular-reference protection.
• Teach render_mjml_component() to handle core/block (not just top-level conversion).
• Add PHPUnit coverage for nested reusable blocks, stale refs, and circular refs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
includes/class-newspack-newsletters-renderer.php	Adds core/block rendering support via resolve_reusable_block(), circular guard stack, and resets stack per render.
tests/test-renderer.php	Adds regression tests covering nested synced patterns, stale refs, and circular references.

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