test(e2e): deterministically wait for event stream in logout propagation test (#38535)

The test raced the server-side SSE channel registration: a logout
emitted before registration is silently dropped ([example
flake](https://github.com/go-gitea/gitea/actions/runs/29699349255/job/88225654080)).
The 500ms wait from https://github.com/go-gitea/gitea/pull/37403 only
made this unlikely.

The server now registers the channel before sending the initial response
bytes, so an open connection implies registration. The shared worker
forwards the built-in `open` event (replaying it to late-attaching ports
via `EventSource.readyState`), the page exposes it as a
`data-user-events-connected` attribute, and the test waits for that
attribute instead of a fixed timeout.

tests/e2e/events.test.ts

@@ -66,12 +66,9 @@ test.describe('events', () => {
     // Verify page2 is logged in
     await expect(page2.getByRole('link', {name: 'Sign In'})).toBeHidden();
 
-    // Give page2's SharedWorker time to register its SSE connection on the
-    // server — otherwise the logout event can race the connection and be
-    // silently dropped. See https://github.com/go-gitea/gitea/pull/37403
-    // In the future, we can set an attribute to HTML page when the connection is established,
-    // then here we can just wait for that attribute (it should also work for the planned WebSocket SharedWorker)
-    await page2.waitForTimeout(500); // eslint-disable-line playwright/no-wait-for-timeout
+    // Wait until the server has registered page2's event stream, otherwise the logout
+    // event can race the connection and be silently dropped.
+    await expect(page2.locator('html[data-user-events-connected]')).toBeAttached();
 
     // Logout from page1 — this sends a logout event to all tabs
     await page1.goto('/user/logout');