Fixes#297
Problem: Tool handlers that return dict/list/None instead of a
JSON string crash the agent loop with cryptic errors. No error
proofing at the boundary.
Fix: In handle_function_call(), after dispatch returns:
1. If result is not str → wrap in JSON with _type_warning
2. If result is str but not valid JSON → wrap in {"output": ...}
3. Log type violations for analysis
4. Valid JSON strings pass through unchanged
Tests: 4 new tests (dict, None, non-JSON string, valid JSON).
All 16 tests in test_model_tools.py pass.
Cover model_tools, toolset_distributions, context_compressor,
prompt_caching, cronjob_tools, session_search, process_registry,
and cron/scheduler with 127 new test cases.
