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_payment_provider_registry.py

@@ -66,6 +66,7 @@ _PROVIDER_MODULES = {
     "wata": "WataService",
     "heleket": "HeleketService",
     "paykilla": "PaykillaService",
+    "lava": "LavaService",
 }
 
 
@@ -128,6 +129,7 @@ def test_service_keys_and_statuses_come_from_provider_specs():
         "cryptopay_service",
         "heleket_service",
         "paykilla_service",
+        "lava_service",
     }
     assert set(pending_statuses()) >= {
         "pending",
@@ -140,6 +142,7 @@ def test_service_keys_and_statuses_come_from_provider_specs():
         "pending_stars",
         "pending_heleket",
         "pending_paykilla",
+        "pending_lava",
     }