Fix flaking signup_spec

Asherah Connorrequested to merge
kiv/fix-flaking-signup-spec into master

What does this MR do and why?

Fix flaking signup_spec.

UsersController#exists is rate-limited at 20 requests/IP/minute! The file's examples type usernames ~30 times, and if CI's too snappy, we get failures.

Disable rate-limiting for the spec to stop it flaking.

See:

Here's an artifact from one of these jobs showing the username validation failing:

image

And the corresponding failure logs:

Failures:
  1) Signup with valid email email: "a@b.example", reason: "valid TLD" accepts emails with valid TLD
     Got 1 failure and 1 other error:
     1.1) Failure/Error: expect(page).to have_selector('[data-testid="new-user-username-field"].gl-field-success-outline')
          
            expected to find css "[data-testid=\"new-user-username-field\"].gl-field-success-outline" but there were no matches
          
            Timeout (30s) reached while running a waiting Capybara finder.
          
            Screenshot: /tmp/capybara/signup_with_valid_email_email_a_b_example_reason_valid_tld_accepts_emails_with_valid_tld.png
            HTML: /tmp/capybara/signup_with_valid_email_email_a_b_example_reason_valid_tld_accepts_emails_with_valid_tld.html
...
     1.2) Failure/Error: raise BrowserConsoleError, message
          
          BrowserConsoleHelpers::BrowserConsoleError:
            Unexpected browser console output:
            http://127.0.0.1:42059/users/user66/exists - Failed to load resource: the server responded with a status of 429 (Too Many Requests)
          
            Screenshot: /tmp/capybara/signup_with_valid_email_email_a_b_example_reason_valid_tld_accepts_emails_with_valid_tld.png
            HTML: /tmp/capybara/signup_with_valid_email_email_a_b_example_reason_valid_tld_accepts_emails_with_valid_tld.html

The browser console error makes the origin clear :)

This has been flaking up to 50 times a day on master pipelines between 2026-09-14 and 2026-09-17, and more than 300 times a day on MR pipelines!

Located with machinery and data from &22879.

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Edited by Asherah Connor

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