feat: implement mintable attribute with irreversible state logic (#26)

### Summary of Changes

- **Added `mintable` attribute**:
  - Enables toggling of minting functionality.
  - Once set to `false`, it cannot be changed back to `true`.

- **Updated `JettonSetParameter` logic**:
  - Ensured `mintable` updates respect the irreversible `false` state.

- **Added Tests**:
  - Verified behavior for `mintable` updates.

---------

Co-authored-by: Kyriakos Andreou <[email protected]>

README.md

@@ -78,4 +78,16 @@ This is default Tact blueprint project with default commands:
             Invalid transfer amount.</br><i>Occurs when you try to send, burn or mint 0 tokens.</i>
         </td>
     </tr>
+        <tr>
+        <td>6906</td>
+        <td>
+            Minting already disabled.</br><i>Occurs when you attempt to enable minting after it has been permanently disabled.</i>
+        </td>
+    </tr>
+        <tr>
+        <td>6907</td>
+        <td>
+            Minting is disabled.</br><i>Occurs when you try to mint tokens while the minting functionality is disabled.</i>
+        </td>
+    </tr>
 </table>

contracts/errors.tact

@@ -9,4 +9,8 @@ const ERROR_JETTON_INITIALIZED: Int = 6903;
 // Jetton max supply exceeded
 const ERROR_MAX_SUPPLY_EXCEEDED: Int = 6904;
 // Invalid transfer amount (e.g., zero tokens)
-const ERROR_CODE_INVALID_AMOUNT: Int = 6905;
+const ERROR_CODE_INVALID_AMOUNT: Int = 6905;
+// Minting already disabled
+const ERROR_CODE_MINTING_ALREADY_DISABLED: Int = 6906; 
+// Minting is disabled
+const ERROR_CODE_MINTING_DISABLED: Int = 6907;

contracts/jetton/master.tact

@@ -12,6 +12,8 @@ contract JettonMaster with TEP74JettonMaster, TEP89JettonDiscoverable, Deployabl
     max_supply: Int = 0;
     // Current tokens minted
     current_supply: Int = 0;
+    // Is token mintable
+    mintable: Bool = true;
     // Administrator of token. Who can mint new tokens
     owner: Address;
     // Initial code of jetton wallet
@@ -59,12 +61,19 @@ contract JettonMaster with TEP74JettonMaster, TEP89JettonDiscoverable, Deployabl
 
             self.max_supply = new_max_supply;
             return;
+        }else if(msg.key == "mintable"){
+            // Once mintable is set to false, it cannot be changed back to true
+            nativeThrowIf(ERROR_CODE_MINTING_ALREADY_DISABLED, !self.mintable);
+
+            self.mintable = msg.value.loadBool();
+            return;
         }
 
         self.metadata.set(msg.key, msg.value); // Update metadata for other keys
     }
 
     receive(msg: JettonMint){
+        nativeThrowIf(ERROR_CODE_MINTING_DISABLED, !self.mintable); // Reject mint if minting is disabled
         nativeThrowUnless(ERROR_CODE_INVALID_AMOUNT, msg.amount > 0); // Reject mint with amount <= 0
         self.requireOwner();