fix: Task.artifacts and Task.history null deserialization (fixes #257) (#258)

# Description

- Initialize artifacts and history fields with empty lists when null
- Add comprehensive unit tests for deserialization scenarios
- Ensures type safety for optional array fields per A2A specification

Fixes #257

Co-authored-by: nirav patel <[email protected]>

Author: Niravkumar-Patel

spec/src/main/java/io/a2a/spec/Task.java

@@ -51,8 +51,8 @@ public Task(@JsonProperty("id") String id, @JsonProperty("contextId") String con
         this.id = id;
         this.contextId = contextId;
         this.status = status;
-        this.artifacts = artifacts;
-        this.history = history;
+        this.artifacts = artifacts != null ? List.copyOf(artifacts) : List.of();
+        this.history = history != null ? List.copyOf(history) : List.of();
         this.metadata = metadata;
         this.kind = kind;
     }

spec/src/test/java/io/a2a/spec/TaskDeserializationTest.java

@@ -0,0 +1,90 @@
+package io.a2a.spec;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class TaskDeserializationTest {
+    private final ObjectMapper objectMapper = new ObjectMapper();
+
+    @Test
+    void testTaskWithMissingHistoryAndArtifacts() throws Exception {
+        // JSON without history and artifacts fields (common server response)
+        String json = """
+            {
+                "id": "task-123",
+                "contextId": "context-456",
+                "status": {
+                    "state": "completed"
+                },
+                "kind": "task"
+            }
+            """;
+
+        Task task = objectMapper.readValue(json, Task.class);
+
+        assertNotNull(task.getHistory(), "history should not be null");
+        assertNotNull(task.getArtifacts(), "artifacts should not be null");
+
+        assertTrue(task.getHistory().isEmpty(), "history should be empty list when not provided");
+        assertTrue(task.getArtifacts().isEmpty(), "artifacts should be empty list when not provided");
+    }
+
+    @Test
+    void testTaskWithExplicitNullValues() throws Exception {
+        // JSON with explicit null values
+        String json = """
+            {
+                "id": "task-123",
+                "contextId": "context-456",
+                "status": {
+                    "state": "completed"
+                },
+                "history": null,
+                "artifacts": null,
+                "kind": "task"
+            }
+            """;
+
+        Task task = objectMapper.readValue(json, Task.class);
+
+        // Should never be null even with explicit null in JSON
+        assertNotNull(task.getHistory(), "history should not be null even when JSON contains null");
+        assertNotNull(task.getArtifacts(), "artifacts should not be null even when JSON contains null");
+
+        assertTrue(task.getHistory().isEmpty());
+        assertTrue(task.getArtifacts().isEmpty());
+    }
+
+    @Test
+    void testTaskWithPopulatedArrays() throws Exception {
+        String json = """
+            {
+                "id": "task-123",
+                "contextId": "context-456",
+                "status": {
+                    "state": "completed"
+                },
+                "history": [
+                    {
+                        "role": "user",
+                        "parts": [{"kind": "text", "text": "hello"}],
+                        "messageId": "msg-1",
+                        "kind": "message"
+                    }
+                ],
+                "artifacts": [],
+                "kind": "task"
+            }
+            """;
+
+        Task task = objectMapper.readValue(json, Task.class);
+
+        assertNotNull(task.getHistory());
+        assertEquals(1, task.getHistory().size());
+
+        assertNotNull(task.getArtifacts());
+        assertTrue(task.getArtifacts().isEmpty());
+    }
+}