perf(pricing): Improve checkout shipping pricing performance by up to 90%

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Summary

• replace random Woo shipping-rate cache busting with a stable checkout-context cache key
• send enabled courier whitelist values to shipping_price instead of an empty courier payload
• reuse shipping_price responses through runtime/session/transient cache so selected-courier fee refreshes do not call the API again
• align plugin AJAX fee-cache context and avoid redundant calculate_totals() calls before Woo update_checkout
• add request-level setting/whitelist caching for repeated checkout setting reads

Validation

• php -l on touched PHP files
• vendor/bin/paratest --configuration paratest.xml --filter CheckoutShippingPerformanceTest
• vendor/bin/paratest --configuration paratest.xml --filter PluginStructure
• vendor/bin/paratest --configuration paratest.xml --filter admin_ajax_fee_refresh_preserves_selected_shipping_method_before_caching_fees
• make zip
• make test still has one unrelated baseline failure: node_modules/flatted/php/flatted.php missing ABSPATH guard in SecurityValidationTest