test(e2e): add pull request merge box test, update AGENTS.md (#38576)

Adds browser-level coverage for the pull request merge box, which
currently had no tests. Exercises the full merge flow: open the PR,
expand the merge form, submit, and assert the merged state.

Extracted from https://github.com/go-gitea/gitea/pull/36759.

Also updated AGENTS.md with instructions for e2e tests.

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

AGENTS.md

@@ -12,7 +12,8 @@
 - Preserve existing code comments, do not remove or rewrite comments that are still relevant
 - Keep comments short, prefer same-line, explain why, never narrate code
 - Prefer unit tests over integration tests when logic is testable in isolation
-- Aim for sub-2s local runtime for integration and e2e tests
+- Aim for sub-2s local runtime per integration test and sub-4s per e2e test
+- Prefer using semantic selectors in e2e tests
 - In TypeScript, use `!` (non-null assertion) instead of `?.`/`??` when a value is known to always exist
 - For CSS layout, prefer `flex-*` helpers over per-child `tw-ml-*` / `tw-mr-*` margins; fall back to `tw-*` utilities when specificity requires `!important`
 - Include authorship attribution in issue and pull request comments

tests/e2e/pull-merge-box.test.ts

@@ -0,0 +1,23 @@
+import {env} from 'node:process';
+import {test, expect} from '@playwright/test';
+import {apiCreateFile, apiCreatePR, apiCreateRepo, assertNoJsError, login, randomString} from './utils.ts';
+
+const owner = env.GITEA_TEST_E2E_USER;
+
+test('merge box merges a pull request', async ({page, request}) => {
+  const repo = `e2e-merge-box-${randomString(8)}`;
+  const createPR = (async () => {
+    await apiCreateRepo(request, {name: repo});
+    await apiCreateFile(request, owner, repo, 'feat.txt', 'feature\n', {branch: 'main', newBranch: 'feat'});
+    return apiCreatePR(request, owner, repo, 'feat', 'main', 'merge box test');
+  })();
+  const [index] = await Promise.all([createPR, login(page)]);
+  await page.goto(`/${owner}/${repo}/pulls/${index}`, {waitUntil: 'commit'});
+
+  // expand the merge form, then submit the merge
+  await page.getByRole('button', {name: 'Create merge commit'}).click();
+  await page.locator('form.form-fetch-action').getByRole('button', {name: 'Create merge commit'}).click();
+
+  await expect(page.getByText(/successfully merged/i)).toBeVisible();
+  await assertNoJsError(page);
+});