Refactor for GRAM (ex TON) integration

- Updated imports and references from TON to GRAM across multiple modules.
- Renamed functions and variables to reflect the change from TON to GRAM.
- Adjusted validation checks for top-up and recharge amounts to use GRAM limits.
- Modified error messages and exceptions to indicate GRAM instead of TON.
- Updated tests to ensure they reflect the new GRAM terminology and functionality.
- Introduced new constants for GRAM-related configurations.

pyfragment/models/enums.py

@@ -15,8 +15,8 @@ else:
 
 
 class PaymentMethod(StrEnum):
-    TON = "ton"
-    USDT_TON = "usdt_ton"
+    GRAM = "ton"
+    USDT_GRAM = "usdt_ton"
 
     # Not supported yet
     USDT_ETH = "usdt_eth"