Enhance Selectel balance handling and error messaging in GPU rental system

- Updated Selectel documentation to clarify error responses related to balance issues.
- Introduced `peek_balance_rub` function for best-effort balance checks, handling API failures gracefully.
- Improved error messages for insufficient funds and quota issues, specifying Selectel's 403 policy response.
- Added balance checks in the doctor command to ensure users are informed about their balance status before attempting GPU creation.
- Refactored exception handling in cloud operations to provide clearer feedback on balance-related errors.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_doctor_balance.py

@@ -0,0 +1,51 @@
+from types import SimpleNamespace
+
+from gpu_rent.doctor import Check, _check_balance
+from gpu_rent.errors import CloudError
+
+
+def test_check_balance_skips_without_token():
+    checks: list[Check] = []
+    _check_balance(SimpleNamespace(selectel_api_token=""), checks)
+    assert checks[0].name == "баланс ₽"
+    assert checks[0].ok is True
+    assert checks[0].blocking is False
+
+
+def test_check_balance_blocks_when_empty(monkeypatch):
+    monkeypatch.setattr("gpu_rent.balance.fetch_balance_rub", lambda *_a, **_k: 0.0)
+    checks: list[Check] = []
+    _check_balance(SimpleNamespace(selectel_api_token="tok"), checks)
+    assert checks[0].ok is False
+    assert checks[0].blocking is True
+    assert "0 ₽" in checks[0].detail
+    assert "Пополни" in checks[0].detail
+
+
+def test_check_balance_warns_when_low(monkeypatch):
+    monkeypatch.setattr("gpu_rent.balance.fetch_balance_rub", lambda *_a, **_k: 40.0)
+    checks: list[Check] = []
+    _check_balance(SimpleNamespace(selectel_api_token="tok"), checks)
+    assert checks[0].ok is True
+    assert checks[0].blocking is False
+    assert "40 ₽" in checks[0].detail
+
+
+def test_check_balance_ok_when_healthy(monkeypatch):
+    monkeypatch.setattr("gpu_rent.balance.fetch_balance_rub", lambda *_a, **_k: 1200.0)
+    checks: list[Check] = []
+    _check_balance(SimpleNamespace(selectel_api_token="tok"), checks)
+    assert checks[0].ok is True
+    assert "1200 ₽" in checks[0].detail
+
+
+def test_check_balance_fetch_error_is_warning(monkeypatch):
+    def boom(*_a, **_k):
+        raise CloudError("сеть")
+
+    monkeypatch.setattr("gpu_rent.balance.fetch_balance_rub", boom)
+    checks: list[Check] = []
+    _check_balance(SimpleNamespace(selectel_api_token="tok"), checks)
+    assert checks[0].ok is True
+    assert checks[0].blocking is False
+    assert "не прочитать" in checks[0].detail