feat: update wallet handling to support separate TON and USDT balances

- Refactor `get_wallet()` to return `ton_balance` and `usdt_balance` in `WalletInfo`.
- Update transaction processing to validate balances for both TON and USDT payment methods.
- Introduce `parse_required_payment_amount` utility to extract payment amounts from responses.
- Modify tests to cover new balance checks and payment method handling.
- Upgrade GitHub Actions artifact upload action to v7.
- Enhance documentation and examples to reflect changes in wallet balance handling.

tests/015_test_payment_amount.py

@@ -0,0 +1,21 @@
+"""Unit tests for init payment amount parsing."""
+
+from pyfragment.utils.html import parse_required_payment_amount
+
+
+def test_parse_required_payment_amount_ton_uses_amount() -> None:
+    init_response = {"amount": "0.326"}
+    assert parse_required_payment_amount(init_response, "ton") == 0.326
+
+
+def test_parse_required_payment_amount_usdt_uses_amount() -> None:
+    init_response = {
+        "amount": "0.00075",
+        "content": '<span class="icon-before icon-usd">0.75</span>',
+    }
+    assert parse_required_payment_amount(init_response, "usdt_ton") == 0.00075
+
+
+def test_parse_required_payment_amount_usdt_falls_back_to_amount() -> None:
+    init_response = {"amount": "1.25", "content": "<p>no usd icon</p>"}
+    assert parse_required_payment_amount(init_response, "usdt_ton") == 1.25