fix: add upstream guard for non-dict function_args + tests for build_tool_preview

Complements PR #453 by 0xbyt4. Adds isinstance(dict) guard in
run_agent.py to catch cases where json.loads returns non-dict
(e.g. null, list, string) before they reach downstream code.

Also adds 15 tests for build_tool_preview covering None args,
empty dicts, known/unknown tools, fallback keys, truncation,
and all special-cased tools (process, todo, memory, session_search).

run_agent.py

@@ -2689,6 +2689,8 @@ class AIAgent:
             except json.JSONDecodeError as e:
                 logging.warning(f"Unexpected JSON error after validation: {e}")
                 function_args = {}
+            if not isinstance(function_args, dict):
+                function_args = {}
 
             if not self.quiet_mode:
                 args_str = json.dumps(function_args, ensure_ascii=False)