test(payments): cover LAVA provider contract

Pin the LAVA integration contract: raw-body HMAC signature in the
Signature header (never a body-embedded field or sorted re-encode),
tolerant webhook signature verification, duplicate-success and
amount-mismatch webhook handling, pending invoice reuse rules, and
RUB-only invoice currency. Extend the registry, wiring, label, and
env-isolation suites with the new provider.

tests/conftest.py

@@ -29,6 +29,7 @@ def _isolate_provider_env(monkeypatch):
                 "SEVERPAY_",
                 "WATA_",
                 "HELEKET_",
+                "LAVA_",
                 "CRYPTOPAY_",
                 "YOOKASSA_",
                 "PAYMENT_FREEKASSA_",
@@ -36,6 +37,7 @@ def _isolate_provider_env(monkeypatch):
                 "PAYMENT_SEVERPAY_",
                 "PAYMENT_WATA_",
                 "PAYMENT_HELEKET_",
+                "PAYMENT_LAVA_",
                 "PAYMENT_CRYPTOPAY_",
                 "PAYMENT_YOOKASSA_",
                 "PAYMENT_STARS_",