fix(gui): use imscope.child in render_heavy_text for exception safety

ROOT CAUSE: render_heavy_text (called per comms panel entry) had
manual begin_child/end_child pairs. If anything inside the child
(especially markdown_helper.render) raised, end_child was skipped.
The child window was left open, corrupting the imGui state. The
corruption cascaded through tab_item.end_tab_item -> tab_bar.end_tab_bar
-> window.end, triggering 'Must call EndChild() and not End()!' assertion.

FIX: Convert the inner begin_child/end_child pair to imscope.child so
the end_child is automatically called by Python's with statement, even
on exception. Also convert prior_scroll to imscope.child for consistency.

TESTS:
- Existing test_comms_no_extraneous_pop.py: push/pop balance check
- Updated test_prior_session_no_clipping.py to match new imscope.child
  signature
- 28/28 broad regression pass

src/gui_2.py

@@ -3589,9 +3589,8 @@ def render_prior_session_view(app: App) -> None:
   imgui.text_colored(vec4(200, 180, 100), f"({len(app.prior_disc_entries)} entries)")
   imgui.separator()
   avail = imgui.get_content_region_avail()
-  with imscope.child("prior_scroll", size_x=avail.x, size_y=avail.y):
-   clipper = imgui.ListClipper(); clipper.begin(len(app.prior_disc_entries))
-   while clipper.step():
+  with imscope.child("prior_scroll", imgui.ImVec2(avail.x, avail.y), imgui.WindowFlags_.horizontal_scrollbar):
+    clipper = imgui.ListClipper(); clipper.begin(len(app.prior_disc_entries))
     for idx in range(clipper.display_start, clipper.display_end):
      entry = app.prior_disc_entries[idx];
      with imscope.id(f"prior_disc_{idx}"):
@@ -4641,19 +4640,15 @@ def render_heavy_text(app: App, label: str, content: str, id_suffix: str = "") -
   ctx_id = f"{label}_{id_suffix}"
   is_md  = label in ('message', 'text', 'content', 'system')
   with imscope.indent():
-   if is_md:
-    imgui.begin_child(f"heavy_text_child_{label}_{id_suffix}", imgui.ImVec2(0, 300), True, imgui.WindowFlags_.always_vertical_scrollbar)
-    markdown_helper.render(content, context_id=ctx_id)
-    imgui.end_child()
-   else:
-    imgui.begin_child(f"heavy_text_child_{label}_{id_suffix}", imgui.ImVec2(0, 200), True, imgui.WindowFlags_.always_vertical_scrollbar)
-    if app.ui_word_wrap:
-     with imscope.text_wrap(imgui.get_content_region_avail().x):
-      imgui.text(content)
+   with imscope.child(f"heavy_text_child_{label}_{id_suffix}", imgui.ImVec2(0, 300), imgui.WindowFlags_.always_vertical_scrollbar):
+    if is_md:
+     markdown_helper.render(content, context_id=ctx_id)
     else:
-     imgui.text(content)
-    imgui.end_child()
-
+     if app.ui_word_wrap:
+      with imscope.text_wrap(imgui.get_content_region_avail().x):
+       imgui.text(content)
+     else:
+      imgui.text(content)
 
 
 #endregion: Misc Tools