fix: use Collectors.toList() for Java 11 compatibility (#6)

Stream.toList() was introduced in Java 16, but the SDK targets Java 11.
This was causing compilation failures on Java 11:

  cannot find symbol
    symbol:   method toList()
    location: interface java.util.stream.Stream<PlanStep>

Author: saurabhjain1592

src/main/java/com/getaxonflow/sdk/AxonFlow.java

@@ -438,7 +438,7 @@ private PlanResponse parsePlanResponse(Response response, String requestDomain)
             if (rawSteps != null) {
                 steps = rawSteps.stream()
                     .map(stepMap -> objectMapper.convertValue(stepMap, PlanStep.class))
-                    .toList();
+                    .collect(java.util.stream.Collectors.toList());
             }
             domain = data.get("domain") != null ? (String) data.get("domain") : domain;
             complexity = data.get("complexity") != null ? ((Number) data.get("complexity")).intValue() : null;