test(rspress-plugin-mermaid): add e2e test with playwright

packages/rspress-plugin-mermaid/index.spec.ts

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+import { test, expect } from '@playwright/test';
+import path from 'path';
+import { fileURLToPath } from 'url';
+import { runDevCommand, killProcess } from '../../e2e/utils.ts';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+
+test.describe('rspress-plugin-mermaid', () => {
+  let devProcess: any;
+  let url: string;
+
+  test.beforeAll(async () => {
+    const result = await runDevCommand(__dirname);
+    devProcess = result.process;
+    url = result.url;
+  });
+
+  test.afterAll(async () => {
+    if (devProcess) {
+      await killProcess(devProcess);
+    }
+  });
+
+  test('should render mermaid diagram', async ({ page }) => {
+    await page.goto(url, { waitUntil: 'networkidle' });
+    // Check if mermaid svg element exists, which is generated by mermaid
+    const mermaidSvg = page.locator('svg[id^="_r_"]');
+    await expect(mermaidSvg).toBeVisible();
+  });
+});

packages/rspress-plugin-mermaid/vitest.config.ts

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+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    exclude: ['**/*.spec.ts', 'node_modules'],
+  },
+});