fix: clean up defensive shims and finish CI stabilization from #17660 (#17801)

PR #17660 landed a sweep of CI fixes but left three loose ends:

1. tests/cli/test_cli_loading_indicator.py::test_reload_mcp_sets_busy_state_
   and_prints_status — /reload-mcp gained a prompt-cache-invalidation
   confirmation (commit 4d7fc0f37) that was never wired into this test.
   The test exercises the loading-indicator path, so pre-approve via
   config and go straight into _reload_mcp().

2. tools/mcp_tool.py _make_tool_handler — the added
   getattr(server, '_rpc_lock', None) + 'skip the lock if missing'
   branch is inconsistent with four sibling call sites that still
   direct-access server._rpc_lock. The lock is guaranteed by
   MCPServerTask.__init__; falling through to an unlocked
   session.call_tool would silently serialize-strip RPCs if the guard
   ever triggered. Restore direct access.

3. tui_gateway/server.py _messages_as_conversation — the helper
   existed only to catch 'TypeError: include_ancestors unexpected'
   from mocked SessionDBs that don't actually exist. The real
   SessionDB.get_messages_as_conversation has accepted
   include_ancestors since introduction, and every test FakeDB in
   the repo already declares the kwarg. Remove the shim, inline the
   two call sites.

tools/mcp_tool.py

@@ -2010,12 +2010,8 @@ def _make_tool_handler(server_name: str, tool_name: str, tool_timeout: float):
             }, ensure_ascii=False)
 
         async def _call():
-            rpc_lock = getattr(server, "_rpc_lock", None)
-            if rpc_lock is None:
+            async with server._rpc_lock:
                 result = await server.session.call_tool(tool_name, arguments=args)
-            else:
-                async with rpc_lock:
-                    result = await server.session.call_tool(tool_name, arguments=args)
             # MCP CallToolResult has .content (list of content blocks) and .isError
             if result.isError:
                 error_text = ""