fix(payments): apply request timeout changes without restart

PAYMENT_REQUEST_TIMEOUT_SECONDS was read once in each provider's
__init__ and baked into the aiohttp session, so admin overrides
(applied in-process) only took effect after a container restart.
Providers now hand HttpClientMixin a timeout source callable; the
mixin builds the session with the current value and swaps in a fresh
session when the value changes, closing the replaced one only after
any in-flight request on it is bound by its own total timeout.

Also:
- check the Heleket payment-info success flag before reading the
  payload so a non-dict provider response cannot raise in the
  pending-payment reuse path
- add PAYMENT_REQUEST_TIMEOUT_SECONDS to the FreeKassa settings stub
  in test_security.py (fixes three tests broken by the new field)

backend/bot/payment_providers/freekassa.py

@@ -156,7 +156,7 @@ class FreeKassaService(HttpClientMixin):
         self.default_currency: str = default_payment_currency_code_for_settings(settings).upper()
 
         self.api_base_url: str = "https://api.fk.life/v1"
-        self._init_http_client(total_timeout=self.settings.PAYMENT_REQUEST_TIMEOUT_SECONDS)
+        self._init_http_client(total_timeout=lambda: self.settings.PAYMENT_REQUEST_TIMEOUT_SECONDS)
         self._nonce_lock = asyncio.Lock()
         self._last_nonce = int(time.time() * 1000)