Issue #3564713: Fix HTML tag parsing to handle whitespace characters in SearchIndex

The search indexing system was using explode(' ') to parse HTML tags, which only splits on space characters. This caused tags with newlines or tabs between the tag name and attributes to be incorrectly parsed, resulting in improper scoring during indexing.

Changed to use preg_split('/\s+/') which properly handles all whitespace characters (spaces, tabs, newlines) as valid separators in HTML tags.

Added test coverage to verify that HTML tags with various whitespace formats (inline, newlines, tabs) are all indexed and scored consistently.

Closes #3564713