Add unit tests for flatUsername in Kotlin and Java test files

Adds enabled unit tests verifying the flatUsername builder method
produces the correct map key and works alongside other attributes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: spetrescu84

msal/src/test/java/com/microsoft/identity/nativeauth/NativeAuthPublicClientApplicationJavaTest.java

@@ -3192,6 +3192,26 @@ public void testSignUpInvalidEmailReturnsError() throws ExecutionException, Inte
         assertTrue(signUpResult instanceof SignUpError);
         assertTrue(((SignUpError) signUpResult).isInvalidUsername());
     }
+
+    @Test
+    public void testUserAttributesFlatUsernameBuilder() {
+        UserAttributes attrs = new UserAttributes.Builder()
+                .flatUsername("testuser")
+                .build();
+        assertEquals("testuser", attrs.getUserAttributes().get("flatusername"));
+    }
+
+    @Test
+    public void testUserAttributesFlatUsernameWithOtherAttributes() {
+        UserAttributes attrs = new UserAttributes.Builder()
+                .flatUsername("testuser")
+                .displayName("Test User")
+                .city("Seattle")
+                .build();
+        assertEquals("testuser", attrs.getUserAttributes().get("flatusername"));
+        assertEquals("Test User", attrs.getUserAttributes().get("displayName"));
+        assertEquals("Seattle", attrs.getUserAttributes().get("city"));
+    }
 }
 
 abstract class TestCallback<T> {

msal/src/test/java/com/microsoft/identity/nativeauth/NativeAuthPublicClientApplicationKotlinTest.kt

@@ -3042,4 +3042,24 @@ class NativeAuthPublicClientApplicationKotlinTest(private val allowPII: Boolean)
         val submitChallengeResult = nextState4.submitChallenge(code)
         assertResult<SignInResult.Complete>(submitChallengeResult)
     }
+
+    @Test
+    fun testUserAttributesFlatUsernameBuilder() {
+        val attrs = UserAttributes.Builder()
+            .flatUsername("testuser")
+            .build()
+        assertEquals("testuser", attrs.userAttributes["flatusername"])
+    }
+
+    @Test
+    fun testUserAttributesFlatUsernameWithOtherAttributes() {
+        val attrs = UserAttributes.Builder()
+            .flatUsername("testuser")
+            .displayName("Test User")
+            .city("Seattle")
+            .build()
+        assertEquals("testuser", attrs.userAttributes["flatusername"])
+        assertEquals("Test User", attrs.userAttributes["displayName"])
+        assertEquals("Seattle", attrs.userAttributes["city"])
+    }
 }