fix: resolve errcheck lint and add missing enable_cch_signing to test

- Suppress errcheck for xxhash Digest.Write (never returns error)
- Add enable_cch_signing field to settings API contract test

backend/internal/service/gateway_billing_header.go

@@ -68,6 +68,6 @@ func signBillingHeaderCCH(body []byte) []byte {
 // xxHash64Seeded computes xxHash64 of data with a custom seed.
 func xxHash64Seeded(data []byte, seed uint64) uint64 {
 	d := xxhash.NewWithSeed(seed)
-	d.Write(data)
+	_, _ = d.Write(data)
 	return d.Sum64()
 }