Refactor form element dispatch - #547

Summary

• Move form element dispatch out of HtmlTransformer::convertElement into FormDispatchTrait
• Preserve existing form, search-form, data-entry fallback, runtime-island, and div-based pseudo-form behavior
• Keep media dispatch untouched to avoid overlap with the recent media refactor

Tests

• composer test from php-transformer

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Implemented the behavior-preserving form dispatch extraction, ran tests, and prepared the PR.