test: widen renderMessages() source window in R16d guard-order probe

The #6717 burst-reset call grows the renderMessages() preamble, pushing the
render-window reset past the probe's 1400-char slice. Widen it to 2400; the
guard-before-reset ordering assertion is unchanged and still passes on master.

tests/test_regressions.py

@@ -793,7 +793,8 @@ def test_renderMessages_preserves_loading_placeholder_for_session_switch(cleanup
     ui_src = (REPO_ROOT / "static/ui.js").read_text()
     fn_start = ui_src.find("function renderMessages")
     assert fn_start >= 0, "renderMessages() not found in ui.js"
-    fn_body = ui_src[fn_start:fn_start + 1400]
+    # Window sized to reach the render-window reset; renderMessages() preamble grows (#6717).
+    fn_body = ui_src[fn_start:fn_start + 2400]
 
     compact = re.sub(r"\s+", "", fn_body)
     assert (