Refactor test helpers and add auth header forwarding tests

Modularizes test helpers into separate files for assertions, generators, and mocks, and updates imports for platform tests. Adds new tests to verify Authorization header forwarding for authenticated requests. Cleans up and simplifies test setup, and updates fixtures for clarity and maintainability.

test/helpers/assertions.js

@@ -0,0 +1,59 @@
+/**
+ * Custom assertions and validation helpers
+ */
+
+/**
+ * Validate response headers for security
+ * @param {Response} response - Response to validate
+ * @returns {Object} Validation results
+ */
+export function validateSecurityHeaders(response) {
+  const requiredHeaders = [
+    'Strict-Transport-Security',
+    'X-Frame-Options',
+    'X-XSS-Protection',
+    'Content-Security-Policy',
+    'Referrer-Policy'
+  ];
+
+  const results = {
+    passed: true,
+    missing: [],
+    present: []
+  };
+
+  requiredHeaders.forEach(header => {
+    if (response.headers.has(header)) {
+      results.present.push(header);
+    } else {
+      results.missing.push(header);
+      results.passed = false;
+    }
+  });
+
+  return results;
+}
+
+/**
+ * Assert that a URL is valid
+ * @param {string} url - URL to validate
+ * @returns {boolean} True if valid
+ */
+export function isValidUrl(url) {
+  try {
+    new URL(url);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Assert that response has security headers
+ * @param {Response} response - Response to check
+ * @returns {boolean} True if has all security headers
+ */
+export function hasSecurityHeaders(response) {
+  const validation = validateSecurityHeaders(response);
+  return validation.passed;
+}