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_balance.py

@@ -1,9 +1,11 @@
 from gpu_rent.balance import (
     BalanceWatchState,
     balance_rub_from_payload,
+    peek_balance_rub,
     plan_balance_notices,
     spend_steps,
 )
+from gpu_rent.errors import CloudError
 
 
 def test_balance_kopecks():
@@ -54,3 +56,17 @@ def test_plan_low_once():
     assert any("низкий" in m for m in msgs)
     state, msgs2 = plan_balance_notices(state, 100.0, step_rub=200.0, low_rub=200.0)
     assert not any("низкий" in m for m in msgs2)
+
+
+def test_peek_balance_rub_empty_token():
+    assert peek_balance_rub("") is None
+    assert peek_balance_rub(None) is None
+
+
+def test_peek_balance_rub_swallows_errors(monkeypatch):
+    def boom(*_a, **_k):
+        raise CloudError("сеть")
+
+    monkeypatch.setattr("gpu_rent.balance.fetch_balance_rub", boom)
+    assert peek_balance_rub("tok") is None
+