Trace unexecuted tool calls in one place; tidy span lifecycles

Paused, denied, skipped and client-run tool calls all go through
trace_unexecuted_tool_call. The research synthesis, continuation and
workflow step spans use with-blocks instead of hand-rolled GeneratorExit
handling. Fixes blank lines and adds a missing hint and docstrings.

docsgpt/storage/db/repositories/request_traces.py

@@ -33,10 +33,12 @@ _SUMMARY_COLUMNS = (
 
 
 def _dump_jsonb(value: Any) -> str:
+    """Serialize ``value`` for a JSONB parameter, without NUL bytes Postgres rejects."""
     return json.dumps(strip_null_bytes(value), cls=PGNativeJSONEncoder)
 
 
 def _uuid_or_none(value: Optional[str]) -> Optional[str]:
+    """``value`` as a string when it is a UUID, else ``None`` (the column is typed)."""
     return str(value) if value and looks_like_uuid(str(value)) else None