Add required validation support for pricing field

[RabbiIslamRony]

Summary

• Add a Required toggle to the Pricing preset field in the Add Listing Form builder, positioned above Admin Only.
• Add safe default pricing required state for migrated/new directory builder data.
• Validate required pricing by meaningful submitted value across Price Unit, Price Range, and Both pricing modes.
• Add frontend required handling so only the visible active pricing control blocks submission, with custom messages for price, range, and no selected pricing option.
• Preserve existing invalid price type and invalid price range validation.

Validation Messages

• Price mode: Price is required.
• Range mode: Price range is required.
• Both mode with no selected option: Please choose a pricing option.
• Backend fallback example: Pricing: Price is required.

Testing

• php -l includes/fields/class-directorist-pricing-field.php
• php -l includes/modules/multi-directory-setup/builder-preset-fields.php
• php -l includes/modules/multi-directory-setup/class-multi-directory-migration.php
• php -l templates/listing-form/fields/pricing.php
• node --check assets/src/js/global/add-listing.js
• git diff --check
• npm run build-legacy passed with existing webpack size/PostCSS warnings.
• Agent Browser cross-test on directory type ID 3:
  • Enabled Pricing Required in builder and confirmed it persisted.
  • Confirmed Required appears on the inner Pricing label.
  • Empty price submission was blocked with Price is required.
  • Backend AJAX fallback returned <strong>Pricing</strong>: Price is required.
  • Submitted a valid test listing with price 123, verified saved pricing data, then trashed the test listing.