test: add unit tests for DeviceCodeOAuthFlow

Add DeviceCodeOAuthFlowTest with tests covering:
- Construction with all fields
- Null refreshUrl (optional field)
- Empty scopes map
- Null validation for required fields (deviceAuthorizationUrl, tokenUrl, scopes)
- Record equality and hashCode

Fixes #607

Author: xyt5442518790

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

@@ -0,0 +1,81 @@
+package io.a2a.spec;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for {@link DeviceCodeOAuthFlow}.
+ * <p>
+ * Tests cover construction with valid parameters, null validation of required
+ * fields, and handling of the optional {@code refreshUrl} field.
+ *
+ * @see DeviceCodeOAuthFlow
+ */
+class DeviceCodeOAuthFlowTest {
+
+    private static final String DEVICE_AUTH_URL = "https://auth.example.com/device/code";
+    private static final String TOKEN_URL = "https://auth.example.com/token";
+    private static final String REFRESH_URL = "https://auth.example.com/refresh";
+    private static final Map<String, String> SCOPES = Map.of("read", "Read access", "write", "Write access");
+
+    @Test
+    void testConstruction_withAllFields() {
+        DeviceCodeOAuthFlow flow = new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, TOKEN_URL, REFRESH_URL, SCOPES);
+
+        assertEquals(DEVICE_AUTH_URL, flow.deviceAuthorizationUrl());
+        assertEquals(TOKEN_URL, flow.tokenUrl());
+        assertEquals(REFRESH_URL, flow.refreshUrl());
+        assertEquals(SCOPES, flow.scopes());
+    }
+
+    @Test
+    void testConstruction_withNullRefreshUrl() {
+        DeviceCodeOAuthFlow flow = new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, TOKEN_URL, null, SCOPES);
+
+        assertEquals(DEVICE_AUTH_URL, flow.deviceAuthorizationUrl());
+        assertEquals(TOKEN_URL, flow.tokenUrl());
+        assertNull(flow.refreshUrl());
+        assertEquals(SCOPES, flow.scopes());
+    }
+
+    @Test
+    void testConstruction_withEmptyScopes() {
+        DeviceCodeOAuthFlow flow = new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, TOKEN_URL, null, Map.of());
+
+        assertNotNull(flow.scopes());
+        assertEquals(0, flow.scopes().size());
+    }
+
+    @Test
+    void testConstruction_nullDeviceAuthorizationUrl_throwsException() {
+        assertThrows(IllegalArgumentException.class,
+                () -> new DeviceCodeOAuthFlow(null, TOKEN_URL, REFRESH_URL, SCOPES));
+    }
+
+    @Test
+    void testConstruction_nullTokenUrl_throwsException() {
+        assertThrows(IllegalArgumentException.class,
+                () -> new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, null, REFRESH_URL, SCOPES));
+    }
+
+    @Test
+    void testConstruction_nullScopes_throwsException() {
+        assertThrows(IllegalArgumentException.class,
+                () -> new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, TOKEN_URL, REFRESH_URL, null));
+    }
+
+    @Test
+    void testEquality_sameValues() {
+        DeviceCodeOAuthFlow flow1 = new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, TOKEN_URL, REFRESH_URL, SCOPES);
+        DeviceCodeOAuthFlow flow2 = new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, TOKEN_URL, REFRESH_URL, SCOPES);
+
+        assertEquals(flow1, flow2);
+        assertEquals(flow1.hashCode(), flow2.hashCode());
+    }
+}