Fix replace_text() destroying nested markup in wrapper blocks

[Copilot]

update_block_text_content() rebuilt innerHTML wholesale from the outermost tag, wiping any nested elements. Calling replace_text() on a core/button block would collapse <div class="wp-block-button"><a href="#">Link Text</a></div> to <div class="wp-block-button">New Text</div> — no <a>, silent data loss.

Changes

• inc/pattern-transformer.php — Rewrites update_block_text_content() to walk tokens via WP_HTML_Tag_Processor::next_token(), find the first non-whitespace #text node, and replace it with set_modifiable_text(). All surrounding markup is preserved untouched.

• Escaping behaviour (intentional breaking change) — replace_text() now HTML-encodes its argument (& → &). Previously it injected text verbatim. Callers needing raw HTML should use the existing replace_html() method.

• Tests — New unit tests for nested-markup preservation and HTML encoding; existing ampersand serialisation test updated to expect the encoded form.

// Before: <a> tag was dropped
$template->replace_text( 'theme/cta', 'core/button', 0, 'Buy Now' );
// → <div class="wp-block-button">Buy Now</div>  ❌

// After: outer div and inner anchor are preserved
// → <div class="wp-block-button"><a class="wp-block-button__link" href="#">Buy Now</a></div>  ✅

• Fixes replace_text() destroys nested markup in wrapper blocks #11