fix: guard model registry lookup and prevent silent json serialization failure

[ELC1657]

Two defensive fixes in the parallel execution path:

tools/streaming.py: _get_agent_token_info() used direct dict
access (ACTIVE_MODEL_INSTANCES[key]) which raises KeyError if a
weakref entry is garbage-collected between the emptiness check and the
lookup. The identical logic in plan.py already uses .get() wrapped
in try/except — this makes streaming.py consistent with that safe
pattern.

parallel_worker.py: json.dump was called without a default
fallback. If any object in message_history is not JSON-serializable,
the TypeError is caught by the file-write error handler and the entire
worker result is silently lost. Added default=str to match the pattern
used in output.py.