refactor(mcp_client,gui_2): migrate ToolDefinition consumers (Phase 8)

Phase 8: ToolDefinition
Before: 2 .get('description',...) sites
After:  0
Delta:  -2 (expected: -2 or -3 per plan; the 3rd site gui_2.py:5875
        is 'server' field which is NOT on ToolDefinition)

Migrates:
1. src/mcp_client.py:1968 (was 1970) - list_tools in _get_tool_definitions:
   tinfo.get('description', '')  ->  ToolDefinition.from_dict(tinfo).description
   (tinfo.get('inputSchema', ...) stays because 'inputSchema' key
    does not match ToolDefinition's 'parameters' field name)

2. src/gui_2.py:5878 - render_external_tools_panel:
   tinfo.get('description', '')  ->  ToolDefinition.from_dict(tinfo).description

Notes:
- gui_2.py:5875 (tinfo.get('server', 'unknown')) is NOT migrated;
  'server' is not a ToolDefinition field. The tinfo here may be a
  ToolInfo or server-info dict, not ToolDefinition. Classified as
  collapsed-codepath per FR2.

Tests: 10/10 pass (test_tool_definition, test_external_mcp,
test_external_mcp_e2e). 2 test_type_aliases failures are pre-existing
(forward references in TypeAlias declarations; not caused by these
changes).

src/gui_2.py

@@ -5875,7 +5875,7 @@ def render_external_tools_panel(app: App) -> None:
     imgui.table_next_column()
     imgui.text(tinfo.get('server', 'unknown'))
     imgui.table_next_column()
-    imgui.text(tinfo.get('description', ''))
+    imgui.text(ToolDefinition.from_dict(tinfo).description if isinstance(tinfo, dict) else tinfo.description)
    imgui.end_table()
  if app.perf_profiling_enabled: app.perf_monitor.end_component("_render_external_tools_panel")
 

src/mcp_client.py

@@ -1965,9 +1965,11 @@ def get_tool_schemas() -> list[dict[str, Any]]:
  res = [t.to_dict() for t in mcp_tool_specs.get_tool_schemas()]
  manager = get_external_mcp_manager()
  for tname, tinfo in manager.get_all_tools().items():
+  from src.type_aliases import ToolDefinition as _TD
+  td = _TD.from_dict(tinfo) if isinstance(tinfo, dict) else tinfo
   res.append({
    'name': tname,
-   'description': tinfo.get('description', ''),
+   'description': td.description,
    'parameters': tinfo.get('inputSchema', {'type': 'object', 'properties': {}})
   })
  return res