Add import behavior controls and improve Substack content conversion

[Ninodevo]

Summary

This PR improves the Substack import flow by adding new import controls.

What changed

• Added new pre-import options to control import behavior:
  • Import all posts as draft.
  • Set the first imported image as featured image.
  • Choose publish date mode (original Substack date vs import date).
  • Apply a global taxonomy term (tag/category) to all imported posts.
• Refactored import options rendering to use a dedicated partial (partials/substack-import-options.php) instead of inline HTML string assembly.
• Updated importer admin logic to process these options and apply behavior consistently during import.
• Improved converter behavior and fixtures for heading/content handling (including expected heading normalization behavior).
• Added/updated PHPUnit coverage for:
  • Import options injection and request sanitization.
  • Import behavior application (status/date/meta/terms/featured-image queue).
  • Converter output expectations and related fixture scenarios.

Test plan

• Run PHPUnit suite and confirm all tests pass.
• Start an import and verify the new Substack Import Options block appears on the pre-import screen.
• Verify each option works:
  • Draft mode imports posts as draft.
  • Featured image option queues/applies first image as thumbnail.
  • Publish date mode switches between original and import-time dates.
  • Global term option assigns selected tag/category to all imported posts.
• Confirm heading conversion output matches expected Gutenberg blocks in imported content.