Wait for DOM selector to exist before interacting with it

The streaming test for 'React Router Sixth Page' was failing with:
  Unable to find css "#ServerComponentRouter-react-component-0"

The issue was that 'within(dom_selector)' was being called before the
selector actually existed in the DOM. For streaming renders, the element
may not appear immediately even with increased wait time.

Fix: Add explicit wait for the CSS selector to exist before trying to
interact with it using within(). This ensures Capybara waits for the
streamed content to fully render before attempting to find child elements.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

react_on_rails_pro/spec/dummy/spec/system/integration_spec.rb

@@ -9,6 +9,9 @@ def change_text_expect_dom_selector(dom_selector, expect_no_change: false)
 
   Capybara.using_wait_time(Capybara.default_max_wait_time * 3) do
     # Streaming renders can take a bit longer for the target node to appear
+    # First ensure the selector exists before trying to interact with it
+    expect(page).to have_css(dom_selector)
+
     within(dom_selector) do
       find("input").set new_text
       within("h3") do