feat(assistant): implement spec 021 chat assistant flow with semantic contracts

frontend/src/lib/stores/__tests__/assistantChat.test.js

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+// [DEF:frontend.src.lib.stores.__tests__.assistantChat:Module]
+// @TIER: STANDARD
+// @SEMANTICS: test, store, assistant, toggle, conversation
+// @PURPOSE: Validate assistant chat store visibility and conversation binding transitions.
+// @LAYER: UI Tests
+// @RELATION: DEPENDS_ON -> assistantChatStore
+// @INVARIANT: Each test starts from default closed state.
+
+import { describe, it, expect, beforeEach } from 'vitest';
+import { get } from 'svelte/store';
+import {
+  assistantChatStore,
+  toggleAssistantChat,
+  openAssistantChat,
+  closeAssistantChat,
+  setAssistantConversationId,
+} from '../assistantChat.js';
+
+// [DEF:assistantChatStore_tests:Function]
+// @TIER: STANDARD
+// @PURPOSE: Group store unit scenarios for assistant panel behavior.
+// @PRE: Store can be reset to baseline state in beforeEach hook.
+// @POST: Open/close/toggle/conversation transitions are validated.
+describe('assistantChatStore', () => {
+  beforeEach(() => {
+    assistantChatStore.set({
+      isOpen: false,
+      conversationId: null,
+    });
+  });
+
+  it('should open assistant panel', () => {
+    openAssistantChat();
+    const state = get(assistantChatStore);
+    expect(state.isOpen).toBe(true);
+  });
+
+  it('should close assistant panel', () => {
+    openAssistantChat();
+    closeAssistantChat();
+    const state = get(assistantChatStore);
+    expect(state.isOpen).toBe(false);
+  });
+
+  it('should toggle assistant panel state', () => {
+    toggleAssistantChat();
+    expect(get(assistantChatStore).isOpen).toBe(true);
+    toggleAssistantChat();
+    expect(get(assistantChatStore).isOpen).toBe(false);
+  });
+
+  it('should set conversation id', () => {
+    setAssistantConversationId('conv-123');
+    const state = get(assistantChatStore);
+    expect(state.conversationId).toBe('conv-123');
+  });
+});
+// [/DEF:assistantChatStore_tests:Function]
+// [/DEF:frontend.src.lib.stores.__tests__.assistantChat:Module]