Update tax_helper to apply tax amount if applicable

Enhanced the tax applicability logic by introducing the `tax_applicable?` method in the `Spree::Tax::TaxHelpers` module.
The method determines if tax is applicable based on the `tax_reverse_charge_mode` of the tax category and the `reverse_charge_status`
of the address. The `rates_for_item` method has been updated to use this new logic, ensuring that only applicable tax rates are selected.

The `tax_applicable?` method supports three modes:
- **strict**: Tax applies only if the address is not enabled.
- **loose**: Tax applies only if the address is explicitly disabled.
- **disabled**: Tax always applies.

Author: JustShah

core/app/models/spree/tax/tax_helpers.rb

@@ -20,9 +20,33 @@ def rates_for_item(item)
         tax_category_id = item.try(:variant_tax_category_id) || item.tax_category_id
 
         @rates_for_item.select do |rate|
+          tax_category = rate.tax_categories.find { |tax_cat| tax_cat.id == tax_category_id }
+          next unless tax_category && tax_applicable?(tax_category, item.order.tax_address)
+
           rate.active? && rate.tax_categories.map(&:id).include?(tax_category_id)
         end
       end
+
+      # Determine if tax is applicable based on tax category and address
+      #
+      # @param [Spree::TaxCategory] tax_category
+      #   the tax category to check tax_reverse_charge_mode
+      # @param [Spree::Address] address
+      #   the address to check reverse_charge_status
+      # @return [Boolean] true if tax is applicable, false otherwise
+      def tax_applicable?(tax_category, address)
+        case tax_category.tax_reverse_charge_mode
+          # Strict mode: Tax applies only if the address is NOT enabled (reverse charge)
+        when 'strict'
+          !address.reverse_charge_status_enabled?
+          # Loose mode: Tax applies only if the address is explicitly disabled
+        when 'loose'
+          address.reverse_charge_status_disabled?
+          # Tax always applies
+        when 'disabled'
+          true
+        end
+      end
     end
   end
 end

core/spec/models/spree/tax/tax_helpers_spec.rb

@@ -80,5 +80,106 @@ def valid_rates(item)
         end
       end
     end
+
+    context "when tax_reverse_charge_mode is strict and address is enabled" do
+      let(:tax_category) { create(:tax_category, tax_reverse_charge_mode: 'strict') }
+
+      before do
+        allow(tax_address).to receive(:reverse_charge_status_enabled?).and_return(true)
+      end
+
+      it "returns an empty array" do
+        expect(subject).to be_empty
+      end
+    end
+
+    context "when tax_reverse_charge_mode is strict and address is not enabled" do
+      let(:tax_category) { create(:tax_category, tax_reverse_charge_mode: 'strict') }
+
+      before do
+        allow(tax_address).to receive(:reverse_charge_status_enabled?).and_return(false)
+      end
+
+      it "returns the applicable tax rate" do
+        expect(subject).to contain_exactly(tax_rate)
+      end
+    end
+
+    context "when tax_reverse_charge_mode is loose and address is disabled" do
+      let(:tax_category) { create(:tax_category, tax_reverse_charge_mode: 'loose') }
+
+      before do
+        allow(tax_address).to receive(:reverse_charge_status_disabled?).and_return(true)
+      end
+
+      it "returns the applicable tax rate" do
+        expect(subject).to contain_exactly(tax_rate)
+      end
+    end
+
+    context "when tax_reverse_charge_mode is loose and address is not disabled" do
+      let(:tax_category) { create(:tax_category, tax_reverse_charge_mode: 'loose') }
+
+      before do
+        allow(tax_address).to receive(:reverse_charge_status_disabled?).and_return(false)
+      end
+
+      it "returns an empty array" do
+        expect(subject).to be_empty
+      end
+    end
+
+    context "when tax_reverse_charge_mode is disabled" do
+      let(:tax_category) { create(:tax_category, tax_reverse_charge_mode: 'disabled') }
+
+      it "returns the applicable tax rate" do
+        expect(subject).to contain_exactly(tax_rate)
+      end
+    end
+  end
+
+  describe '#tax_applicable?' do
+    let(:tax_category) { create(:tax_category, tax_reverse_charge_mode: tax_reverse_charge_mode) }
+    let(:address) { create(:address) }
+
+    subject { DummyClass.new.send(:tax_applicable?, tax_category, address) }
+
+    context 'when tax_reverse_charge_mode is strict' do
+      let(:tax_reverse_charge_mode) { 'strict' }
+
+      context 'when address is enabled' do
+        before { allow(address).to receive(:reverse_charge_status_enabled?).and_return(true) }
+
+        it { is_expected.to be_falsey }
+      end
+
+      context 'when address is not enabled' do
+        before { allow(address).to receive(:reverse_charge_status_enabled?).and_return(false) }
+
+        it { is_expected.to be_truthy }
+      end
+    end
+
+    context 'when tax_reverse_charge_mode is loose' do
+      let(:tax_reverse_charge_mode) { 'loose' }
+
+      context 'when address is disabled' do
+        before { allow(address).to receive(:reverse_charge_status_disabled?).and_return(true) }
+
+        it { is_expected.to be_truthy }
+      end
+
+      context 'when address is not disabled' do
+        before { allow(address).to receive(:reverse_charge_status_disabled?).and_return(false) }
+
+        it { is_expected.to be_falsey }
+      end
+    end
+
+    context 'when tax_reverse_charge_mode is disabled' do
+      let(:tax_reverse_charge_mode) { 'disabled' }
+
+      it { is_expected.to be_truthy }
+    end
   end
 end