Extract background image parsing helper

[chubes4]

Summary

• Extract CSS background image URL parsing and alt derivation into a focused PHP support helper.
• Keep HtmlTransformer responsible for creating the image block and resolving asset metadata URLs.
• No JS package edits.

Tests

• composer test

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Architecture hotspot inspection, PHP-only refactor extraction, test verification, and PR preparation.