Make Trace class inherit from abc.ABC for proper abstract base class behavior (openai#1233)

The Trace class was using @abc.abstractmethod decorators without
inheriting
from abc.ABC, which meant the abstract methods weren't enforced.

This change makes the class properly abstract while maintaining all
existing functionality
since no code directly instantiates Trace() - all usage goes through the
concrete implementations NoOpTrace and TraceImpl.

Author: DanielHashmi

src/agents/tracing/traces.py

@@ -10,7 +10,7 @@
 from .scope import Scope
 
 
-class Trace:
+class Trace(abc.ABC):
     """
     A trace is the root level object that tracing creates. It represents a logical "workflow".
     """