feat: ChargeDiscountWallet - callback endpoint, gRPC RPC & service implementation

- Add /api/wallet/verify-discount-charge callback endpoint in PaymentCallbackController
- Add InitiateDiscountCharge RPC + request/response messages in userwallet.proto
- Add InitiateDiscountCharge gRPC service override in UserWalletService
- Add ChargeDiscountWallet using import for handler resolution

src/CMSMicroservice.Protobuf/Protos/userwallet.proto

@@ -88,6 +88,15 @@ service UserWalletContract
             get: "/Customer/GetMagicWalletStatus"
         };
     };
+
+    // ============= Discount Wallet Methods =============
+
+    rpc InitiateDiscountCharge(InitiateDiscountChargeRequest) returns (InitiateDiscountChargeResponse){
+        option (google.api.http) = {
+            post: "/Customer/InitiateDiscountCharge"
+            body: "*"
+        };
+    };
 }
 message CreateNewUserWalletRequest
 {
@@ -241,4 +250,18 @@ message GetMagicWalletStatusResponse
     int64 balance = 6;
     google.protobuf.Timestamp magic_activated_at = 7;
     int32 purchase_cycle_count = 8; // تعداد دورهای خرید پکیج (0 = هنوز هیچ دوری تکمیل نشده)
+}
+
+// ============= Discount Wallet Messages =============
+
+message InitiateDiscountChargeRequest
+{
+    int64 amount = 1; // مبلغ واریزی (ریال)
+}
+
+message InitiateDiscountChargeResponse
+{
+    bool is_success = 1;
+    string gateway_url = 2;
+    string error_message = 3;
 }