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

tests/test_prior_session_no_clipping.py

@@ -5,8 +5,10 @@ def test_prior_session_view_opens_scroll_child_with_explicit_size(app_instance):
  app_instance.prior_disc_entries = [{"role": "User", "content": f"entry {i}", "collapsed": False, "ts": f"t{i}"} for i in range(30)]
  app_instance.perf_profiling_enabled = False
  with patch("src.gui_2.imgui") as mock_imgui, patch("src.gui_2.imscope") as mock_imscope, patch("src.gui_2.theme") as mock_theme:
-  _avail = MagicMock(); _avail.x = 800.0; _avail.y = 600.0
+  _avail = type("P", (), {"x": 800.0, "y": 600.0})()
   mock_imgui.get_content_region_avail = MagicMock(return_value=_avail)
+  def _imvec2(x, y=0): m = MagicMock(); m.x = float(x); m.y = float(y); return m
+  mock_imgui.ImVec2 = _imvec2
   mock_imscope.style_color = MagicMock()
   mock_imscope.style_color.return_value.__enter__ = MagicMock()
   mock_imscope.style_color.return_value.__exit__ = MagicMock()
@@ -38,6 +40,12 @@ def test_prior_session_view_opens_scroll_child_with_explicit_size(app_instance):
    pytest.fail(f"render_prior_session_view raised: {e}")
  assert mock_imscope.child.called, "prior_scroll child should be opened"
  call_args = mock_imscope.child.call_args
+ args_list = call_args[0] if call_args[0] else []
  kwargs = call_args[1] if len(call_args) > 1 else {}
- size_x, size_y = kwargs.get("size_x", 0), kwargs.get("size_y", 0)
+ size_arg = args_list[1] if len(args_list) > 1 else kwargs.get("size_x", 0)
+ if hasattr(size_arg, 'x'):
+  size_x, size_y = size_arg.x, size_arg.y
+ else:
+  size_x = size_arg
+  size_y = kwargs.get("size_y", 0)
  assert size_x == 800.0 and size_y == 600.0, f"prior_scroll should use explicit content region size, got ({size_x}, {size_y})"