fix: tighten chat selection state invariants

android/app/src/main/java/com/hyzq/boss/ProjectChatUiState.java

@@ -2,6 +2,7 @@ package com.hyzq.boss;
 
 import androidx.annotation.Nullable;
 
+import java.util.Collections;
 import java.util.LinkedHashSet;
 import java.util.Set;
 
@@ -12,9 +13,10 @@ public final class ProjectChatUiState {
         public final boolean multiSelecting;
         public final Set<String> selectedMessageIds;
 
-        public SelectionState(boolean multiSelecting, Set<String> selectedMessageIds) {
-            this.multiSelecting = multiSelecting;
-            this.selectedMessageIds = new LinkedHashSet<>(selectedMessageIds);
+        private SelectionState(Set<String> selectedMessageIds) {
+            LinkedHashSet<String> normalizedIds = new LinkedHashSet<>(selectedMessageIds);
+            this.multiSelecting = !normalizedIds.isEmpty();
+            this.selectedMessageIds = Collections.unmodifiableSet(normalizedIds);
         }
     }
 
@@ -27,21 +29,24 @@ public final class ProjectChatUiState {
     }
 
     public static SelectionState emptySelection() {
-        return new SelectionState(false, new LinkedHashSet<>());
+        return new SelectionState(new LinkedHashSet<>());
     }
 
     public static SelectionState toggleSelection(@Nullable SelectionState current, String messageId) {
+        if (messageId == null || messageId.trim().isEmpty()) {
+            throw new IllegalArgumentException("messageId must not be blank");
+        }
         SelectionState state = current == null ? emptySelection() : current;
         LinkedHashSet<String> selectedMessageIds = new LinkedHashSet<>(state.selectedMessageIds);
         if (selectedMessageIds.contains(messageId)) {
             selectedMessageIds.remove(messageId);
-            return new SelectionState(!selectedMessageIds.isEmpty(), selectedMessageIds);
+            return new SelectionState(selectedMessageIds);
         }
         selectedMessageIds.add(messageId);
-        return new SelectionState(true, selectedMessageIds);
+        return new SelectionState(selectedMessageIds);
     }
 
     public static boolean canForwardSelection(@Nullable SelectionState state) {
-        return state != null && state.selectedMessageIds.size() >= 2;
+        return state != null && state.multiSelecting && state.selectedMessageIds.size() >= 2;
     }
 }

android/app/src/test/java/com/hyzq/boss/ProjectChatUiStateTest.java

@@ -1,8 +1,12 @@
 package com.hyzq.boss;
 
+import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+
+import java.util.ArrayList;
 
 import org.junit.Test;
 
@@ -32,8 +36,7 @@ public class ProjectChatUiStateTest {
 
     @Test
     public void deselectingLastMessageExitsMultiSelectMode() {
-        ProjectChatUiState.SelectionState state =
-                new ProjectChatUiState.SelectionState(true, java.util.Set.of("m1"));
+        ProjectChatUiState.SelectionState state = ProjectChatUiState.toggleSelection(null, "m1");
         ProjectChatUiState.SelectionState next = ProjectChatUiState.toggleSelection(state, "m1");
         assertFalse(next.multiSelecting);
         assertTrue(next.selectedMessageIds.isEmpty());
@@ -41,8 +44,32 @@ public class ProjectChatUiStateTest {
 
     @Test
     public void bundleForwardRequiresAtLeastTwoMessages() {
-        ProjectChatUiState.SelectionState state =
-                new ProjectChatUiState.SelectionState(true, java.util.Set.of("m1"));
+        ProjectChatUiState.SelectionState state = ProjectChatUiState.toggleSelection(null, "m1");
         assertFalse(ProjectChatUiState.canForwardSelection(state));
+
+        ProjectChatUiState.SelectionState next = ProjectChatUiState.toggleSelection(state, "m2");
+        assertTrue(ProjectChatUiState.canForwardSelection(next));
+    }
+
+    @Test
+    public void selectionPreservesInsertionOrder() {
+        ProjectChatUiState.SelectionState state = ProjectChatUiState.toggleSelection(null, "m2");
+        state = ProjectChatUiState.toggleSelection(state, "m1");
+        state = ProjectChatUiState.toggleSelection(state, "m3");
+
+        assertArrayEquals(
+                new String[] {"m2", "m1", "m3"},
+                new ArrayList<>(state.selectedMessageIds).toArray(new String[0])
+        );
+    }
+
+    @Test
+    public void toggleSelectionRejectsBlankMessageIds() {
+        try {
+            ProjectChatUiState.toggleSelection(null, "   ");
+            fail("Expected IllegalArgumentException");
+        } catch (IllegalArgumentException expected) {
+            assertEquals("messageId must not be blank", expected.getMessage());
+        }
     }
 }