Preserve whitespace inside pre elements when federating content

[obenland]

Fixes #2592

Proposed changes:

• Add Sanitize::strip_whitespace() method that strips newlines, carriage returns, and tabs from content while preserving them inside <pre> elements.
• Update Shortcodes::content() and Post::get_content() to use the new sanitization method.

Code blocks in federated posts were losing their newline characters, making them unreadable. The fix uses a regex with preg_replace_callback to preserve whitespace inside <pre> tags while stripping it from the rest of the content.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

1. Create a post with a code block containing multiple lines
2. Publish the post
3. View the federated content on a remote instance (or inspect the ActivityPub JSON at /wp-json/activitypub/1.0/posts/{id})
4. Verify the code block preserves newlines and formatting

Changelog entry

• Automatically create a changelog entry from the details below.

Changelog Entry Details

Significance

• Patch
• Minor
• Major

Type

• Added - for new features
• Changed - for changes in existing functionality
• Deprecated - for soon-to-be removed features
• Removed - for now removed features
• Fixed - for any bug fixes
• Security - in case of vulnerabilities

Message

Preserve newlines inside pre elements when federating content.

[Copilot]

Pull request overview

This PR fixes whitespace handling in federated content by preserving formatting inside <pre> elements while stripping unnecessary whitespace elsewhere. Previously, code blocks were losing their newlines during federation, making them unreadable.

Key Changes:

• Added Sanitize::strip_whitespace() method to handle selective whitespace stripping
• Updated content processing in Post::get_content() and Shortcodes::content() to use the new method
• Added comprehensive test coverage for both isolated code blocks and mixed content scenarios

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
includes/class-sanitize.php	Introduces the new strip_whitespace() method that uses regex callback to preserve whitespace in <pre> tags
includes/transformer/class-post.php	Replaces direct regex whitespace stripping with the new sanitization method
includes/class-shortcodes.php	Updates shortcode content processing to use the new sanitization method
tests/phpunit/tests/includes/transformer/class-test-post.php	Adds two test cases validating whitespace preservation in code blocks
.github/changelog/2595-from-description	Changelog entry documenting the fix

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