Forms: Migrate form fields to inner blocks

[aaronrobertshaw]

Part of: #42353
Related: #41281
Project Thread: TBA

Description

This issue finalizes the switch to form field inner blocks by introducing block deprecations that transform each form field into the new structure. We’ll also update the style syncing logic and shortcode rendering to handle child blocks correctly.

Scope & Requirements

• Implement block deprecations mapping old attributes (label text, input type, etc.) into new Label, Input, and Option blocks.
• Update shortcode rendering functions to collect styling data from nested blocks and apply it to generated HTML.
• Ensure style syncing (if enabled) is compatible with multiple child blocks within a single field.

Tasks

• Add block deprecations
  • For each current field type, define a deprecation transforming it into a parent container with inner blocks.
  • Test with posts that contain older forms to confirm a smooth upgrade experience.
• Update shortcode rendering
  • Map nested child block attributes to the expected markup or classes.
  • Confirm frontend display remains consistent for non-migrated and migrated blocks alike
• Refactor style syncing
  • Refactor existing HoC to a new custom hook that handles style syncing in a manner compatible with child blocks.
• Final tests & validation
  • Add or update automated tests to confirm no regressions.
  • Check multiple field scenarios (text, email, radio, checkbox) in both old and new states.