feat(events): widen content type to str|dict in all store backends

Allow event content to be a dict (for structured OpenAI-format messages)
in addition to plain strings. Dict values are JSON-serialized for the DB
backend and deserialized on read; memory and JSONL backends handle dicts
natively. Trace truncation now serializes dicts to JSON before measuring.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/packages/harness/deerflow/runtime/events/store/base.py

@@ -33,7 +33,7 @@ class RunEventStore(abc.ABC):
         run_id: str,
         event_type: str,
         category: str,
-        content: str = "",
+        content: str | dict = "",
         metadata: dict | None = None,
         created_at: str | None = None,
     ) -> dict: