test(react): add tests for useKatexStyles hook (CopilotKit#3397)

## Summary

Follow-up to CopilotKit#3314. Adds test coverage for the `useKatexStyles` hook:

1. **Renders without error** — dynamic CSS import succeeds
2. **Graceful failure** — doesn't throw when katex CSS import fails
3. **Regression guard** — reads the component source to ensure the
static `import 'katex/dist/katex.min.css'` doesn't creep back in

## Test plan
- [x] `nx run @copilotkitnext/react:test -- use-katex-styles` — 3/3 pass

Author: jpr5

packages/v2/react/src/hooks/__tests__/use-katex-styles.test.tsx

@@ -0,0 +1,56 @@
+import { renderHook } from "@testing-library/react";
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+// Mock the katex CSS import globally
+vi.mock("katex/dist/katex.min.css", () => ({}));
+
+describe("useKatexStyles", () => {
+  beforeEach(() => {
+    // Reset module state (the singleton `injected` flag) between tests
+    vi.resetModules();
+  });
+
+  async function loadHook() {
+    const mod = await import("../useKatexStyles");
+    return mod.useKatexStyles;
+  }
+
+  it("renders without error (dynamic import succeeds)", async () => {
+    const useKatexStyles = await loadHook();
+
+    expect(() => {
+      renderHook(() => useKatexStyles());
+    }).not.toThrow();
+  });
+
+  it("does not throw when katex CSS import fails", async () => {
+    vi.doMock("katex/dist/katex.min.css", () => {
+      throw new Error("CSS not found");
+    });
+
+    const useKatexStyles = await loadHook();
+
+    expect(() => {
+      renderHook(() => useKatexStyles());
+    }).not.toThrow();
+  });
+
+  it("does not use a static katex CSS import in the component", async () => {
+    // Regression guard: ensure the static import doesn't creep back.
+    // Read the component source and verify no static katex CSS import.
+    const fs = await import("fs");
+    const path = await import("path");
+    const componentPath = path.resolve(
+      __dirname,
+      "../../components/chat/CopilotChatAssistantMessage.tsx",
+    );
+    const source = fs.readFileSync(componentPath, "utf-8");
+
+    // Should NOT have a static import of katex CSS
+    expect(source).not.toMatch(
+      /^import\s+['"]katex\/dist\/katex\.min\.css['"]/m,
+    );
+    // Should use the hook instead
+    expect(source).toMatch(/useKatexStyles/);
+  });
+});