fix(tracing): Deep-copy spans before enqueuing to background queue

Processors like SGPAsyncTracingProcessor mutate span.data in-place
via _add_source_to_span. With async background processing, this raced
with the caller who holds a reference to the same span object.
Deep-copying via model_copy(deep=True) decouples the two.

Author: smoreinis

src/agentex/lib/core/tracing/trace.py

@@ -232,7 +232,7 @@ async def start_span(
         )
 
         if self.processors:
-            self._span_queue.enqueue(SpanEventType.START, span, self.processors)
+            self._span_queue.enqueue(SpanEventType.START, span.model_copy(deep=True), self.processors)
 
         return span
 
@@ -257,7 +257,7 @@ async def end_span(
         span.data = recursive_model_dump(span.data) if span.data else None
 
         if self.processors:
-            self._span_queue.enqueue(SpanEventType.END, span, self.processors)
+            self._span_queue.enqueue(SpanEventType.END, span.model_copy(deep=True), self.processors)
 
         return span