Fix QA Duo Chat page object: update submit button testid

What does this MR do?

Updates the QA page object qa/qa/ee/page/component/duo_chat.rb to use the new submit button data-testid (chat-prompt-submit-button) instead of the removed paper-airplane-icon.

Why is this needed?

In !235880 (merged) ("Refine Duo Chat panel spacing and border radii"), the Duo Chat submit icon was switched from paper-airplane to arrow-up, and the data-testid was renamed from paper-airplane-icon to chat-prompt-submit-button. The QA page object still referenced the old testid, causing every E2E test that sends a Duo Chat prompt (including clear_chat_history, which sends /clear) to fail with:

Capybara::ElementNotFound:
  Unable to find css "[data-testid=\"paper-airplane-icon\"]"

Related

• Test failure issue: gitlab-org/quality/test-failure-issues#42889
• Original UI change: !235880 (merged)
• Failing spec: qa/qa/specs/features/ee/browser_ui/4_verify/root_cause_analysis_with_duo_chat_spec.rb

How to verify

Run the affected spec (and any other Duo Chat E2E specs) against an environment with the new submit button.