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/test_build_services_wiring.py

@@ -35,6 +35,7 @@ _PROVIDER_ENV_PREFIXES = (
     "SEVERPAY_",
     "WATA_",
     "HELEKET_",
+    "LAVA_",
     "CRYPTOPAY_",
     "YOOKASSA_",
     "STARS_",
@@ -176,6 +177,7 @@ class BuildServicesWiringTests(unittest.TestCase):
             "wata_service",
             "heleket_service",
             "paykilla_service",
+            "lava_service",
         }
         self.assertEqual(set(services), expected_keys)