fix(response-viewer): the Copy Markdown button is named by the words on it, and the full view's fallback has a test (#810)

The button's aria-label replaced its visible text as the accessible name with
a sentence that does not contain "Copy Markdown" (WCAG 2.5.3, Label in Name),
so a voice-control user could not say what they see. The label is gone and the
sentence stays as the title.

No test reached the line that keeps the brief view's answer when the full view
holds no answer of its own: the existing case returns earlier, on an empty
list. A full view that holds only a prompt reaches it.

The PR's own changeset is folded into the 1.51.0 one.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

.changeset/response-copy-markdown.md

@@ -1,5 +0,0 @@
----
-"aicodeman": patch
----
-
-Response viewer: add a Copy Markdown button that copies the raw source of the newest answer (every part of its turn), so headings, lists and code fences paste intact. An empty viewer and a refused clipboard both say so.