feat: Replace custom JS form refresh with Drupal core HTMX support

Martin Anderson-Clutzrequested to merge
issue/canvas_field_component-3587445:feature/htmx-form-refresh into 1.0.x

Removes the Redux-polling JavaScript in favour of declarative HTMX attributes backed by a new Drupal form endpoint. When the field or formatter select changes, HTMX POSTs to the new /canvas-field-component/htmx/formatter-section route and swaps the formatter section container (formatter select, label display, and settings) in place, eliminating the need for any custom JavaScript.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com and Codex (ChatGPT 5.5)

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