test: add eval targeting fill_form use with select and checkbox elements (#1968)

This eval represents minimal reproduction scenario for #1942. Currently
fill_form is used in around 25% of the attempts.

It can also be used to reproduce #1941 with about 1:10 chance.

---------

Co-authored-by: Piotr Paulski <piotrpaulski@chromium.org>

Author: zyzyzyryxy

scripts/eval_scenarios/fill_select_and_checkboxes_test.ts

@@ -0,0 +1,67 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import assert from 'node:assert';
+
+import type {TestScenario} from '../eval_gemini.ts';
+
+export const scenario: TestScenario = {
+  prompt:
+    'Go to <TEST_URL>, fill the form with size = 2 CPUs and components = [docker, nginx].',
+  maxTurns: 3,
+  htmlRoute: {
+    path: '/input_test.html',
+    htmlContent: `
+      <form action="/post" method="POST">
+        <div>
+          <label for="size">CPU/Memory size:</label>
+          <select id="size" name="size" required>
+            <option value="small">1 vCPU, 2GB RAM</option>
+            <option value="medium">2 vCPU, 4GB RAM</option>
+            <option value="large">4 vCPU, 8GB RAM</option>
+          </select>
+        </div>
+        <br>
+        <div>
+          <p>Pre-installed components:</p>
+          <input type="checkbox" id="docker" name="components" value="docker">
+          <label for="docker">Docker</label><br>
+          <input type="checkbox" id="nodejs" name="components" value="nodejs">
+          <label for="nodejs">Node.js</label><br>
+          <input type="checkbox" id="python" name="components" value="python">
+          <label for="python">Python</label><br>
+          <input type="checkbox" id="nginx" name="components" value="nginx">
+          <label for="nginx">Nginx</label>
+        </div>
+        <button type="submit">Spawn Server</button>
+      </form>
+    `,
+  },
+  expectations: calls => {
+    assert.strictEqual(calls.length, 3);
+    assert.ok(
+      calls[0].name === 'navigate_page' || calls[0].name === 'new_page',
+    );
+    assert.strictEqual(calls[1].name, 'take_snapshot');
+    assert.strictEqual(calls[2].name, 'fill_form');
+
+    const elements = calls[2].args.elements as Array<{
+      uid: string;
+      value: string;
+    }>;
+    assert.strictEqual(elements.length, 3);
+
+    const uids = new Set(elements.map(e => e.uid));
+    assert.strictEqual(
+      uids.size,
+      3,
+      'fill_form should target three distinct elements',
+    );
+
+    const values = elements.map(e => e.value).sort();
+    assert.deepStrictEqual(values, ['2 vCPU, 4GB RAM', 'true', 'true']);
+  },
+};