feat: Enhance Electron test harness with preload script and CSP configuration

test/electron/preload.js

@@ -0,0 +1,31 @@
+const { contextBridge } = require('electron');
+const fs = require('fs');
+const path = require('path');
+const { pathToFileURL } = require('url');
+
+const pluginManagerBrowserEntryPath = path.join(
+  __dirname,
+  'node_modules',
+  '@paarrot',
+  'plugin-manager',
+  'dist',
+  'index.browser.js'
+);
+const examplePluginPath = path.join(__dirname, 'plugins', 'example-plugin', 'index.js');
+
+/**
+ * Provides the small Node-backed surface area the test harness needs while the
+ * renderer stays in an isolated browser context.
+ */
+contextBridge.exposeInMainWorld('electronTestApi', {
+  getHarnessBootstrap() {
+    return {
+      pluginManagerEntryUrl: pathToFileURL(pluginManagerBrowserEntryPath).href,
+      examplePluginPath,
+      examplePluginCode: fs.readFileSync(examplePluginPath, 'utf-8'),
+    };
+  },
+  async readTextFile(filePath) {
+    return fs.promises.readFile(filePath, 'utf-8');
+  },
+});