Fix incorrect slot name on SpeechSpanData (#386)

Slot had inconsistent name. 

`"first_byte_at"` -> `"first_content_at"`

Author: tcdent

src/agents/tracing/span_data.py

@@ -236,7 +236,7 @@ def export(self) -> dict[str, Any]:
 
 
 class SpeechSpanData(SpanData):
-    __slots__ = ("input", "output", "model", "model_config", "first_byte_at")
+    __slots__ = ("input", "output", "model", "model_config", "first_content_at")
 
     def __init__(
         self,