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_provider_labels.py

@@ -26,6 +26,7 @@ _PROVIDERS_FROM_CALLERS = {
     "platega",
     "severpay",
     "wata",
+    "lava",
     "cryptopay",
     "paykilla",
     "telegram_stars",