External links: screen-reader hint on links that leave the blog

External links in post bodies already opened in a new tab and showed an
icon, but the icon is invisible to screen readers, so nothing announced
that the link leaves the site.

- blogex: ExternalLinks.rewrite/1 appends
  <span class="blogex-external-hint"> (opens in a new tab)</span> as the
  last child of every external anchor. No aria-label, so the accessible
  name still contains the visible link text (WCAG 2.5.3); the icon colour
  and 2.5.3 both hold. Idempotent: a second pass adds no second hint.
- blogex: anchors are now matched together with their closing tag
  (the hint goes inside), so a stray unclosed <a> is left alone.
- app: sr-only rule for .blogex-external-hint in .blogex-post-body. The
  split stays as before: blogex marks up, the host app styles.
- tests: hint cases in blogex (appended, per link, after inner markup,
  idempotent, not on internal/mailto/anchor links) + anchors without href
  (Blogex.ExternalLinks now at 100% line coverage), and an app test that
  the hint reaches the rendered post page. Two exact-string expectations
  updated for the intended markup change.

make test: app 179, blogex 162. make verify: green.

app/test/firehose_web/controllers/blog_test.exs

@@ -79,6 +79,13 @@ defmodule FirehoseWeb.BlogTest do
 
       assert body =~ "target=\"_blank\" rel=\"noopener\" class=\"blogex-external\""
     end
+
+    test "rendered post page carries the screen-reader hint for external links", %{conn: conn} do
+      body =
+        conn |> get("/blog/engineering/delayed-gratification-in-ai-summer") |> html_response(200)
+
+      assert body =~ ~s|<span class="blogex-external-hint"> (opens in a new tab)</span>|
+    end
   end
 
   describe "input validation" do