Add fixed HMAC test vector for auth validation

Adds a test with a pre-computed HMAC value (cross-checked against
Python's hmac module) so the auth logic is verified against a
known fixture, not just round-tripped.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: benthecarman

ldk-server/src/service.rs

@@ -521,6 +521,12 @@ mod tests {
 		assert_eq!(result.unwrap_err().error_code, LdkServerErrorCode::AuthError);
 	}
 
+	#[test]
+	fn test_hmac_known_vector() {
+		let hmac = compute_hmac("test_api_key", 1_000_000_000);
+		assert_eq!(hmac, "3c1a417dd9d2079f0bb5118b4fbb0cc25c695bb6d0e310c2da6cf7dd82f0c9b8");
+	}
+
 	#[test]
 	fn test_validate_auth_expired_timestamp() {
 		let timestamp =