test: enhance DeviceCodeOAuthFlowTest per code review feedback

- Complete field assertions in testConstruction_withEmptyScopes for consistency
- Rename testEquality_sameValues to testEqualityAndHashCode
- Add inequality assertion (assertNotEquals) for different field values
- Replace assertNotNull with assertNotEquals import

This fixes #607

Author: neo1027144-creator

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

@@ -1,7 +1,7 @@
 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.assertNotEquals;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
@@ -46,10 +46,13 @@ void testConstruction_withNullRefreshUrl() {
 
     @Test
     void testConstruction_withEmptyScopes() {
-        DeviceCodeOAuthFlow flow = new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, TOKEN_URL, null, Map.of());
+        Map<String, String> emptyScopes = Map.of();
+        DeviceCodeOAuthFlow flow = new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, TOKEN_URL, null, emptyScopes);
 
-        assertNotNull(flow.scopes());
-        assertEquals(0, flow.scopes().size());
+        assertEquals(DEVICE_AUTH_URL, flow.deviceAuthorizationUrl());
+        assertEquals(TOKEN_URL, flow.tokenUrl());
+        assertNull(flow.refreshUrl());
+        assertEquals(emptyScopes, flow.scopes());
     }
 
     @Test
@@ -71,11 +74,16 @@ void testConstruction_nullScopes_throwsException() {
     }
 
     @Test
-    void testEquality_sameValues() {
+    void testEqualityAndHashCode() {
         DeviceCodeOAuthFlow flow1 = new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, TOKEN_URL, REFRESH_URL, SCOPES);
         DeviceCodeOAuthFlow flow2 = new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, TOKEN_URL, REFRESH_URL, SCOPES);
+        DeviceCodeOAuthFlow flow3WithNullRefresh = new DeviceCodeOAuthFlow(DEVICE_AUTH_URL, TOKEN_URL, null, SCOPES);
 
+        // Test for equality with same values
         assertEquals(flow1, flow2);
         assertEquals(flow1.hashCode(), flow2.hashCode());
+
+        // Test for inequality with different values
+        assertNotEquals(flow1, flow3WithNullRefresh);
     }
 }