Fix: Enter Langfuse span context to properly attach child spans

The Langfuse session span was created with start_as_current_span() but
never entered, causing child generations and tool spans to be orphaned.
Now properly using __enter__()/__exit__() to manage span lifecycle.

This fixes the missing Langfuse SDK traces in the UI.

Author: sallyom

components/runners/claude-code-runner/observability.py

@@ -121,6 +121,8 @@ async def _init_langfuse(self, prompt: str, namespace: str) -> bool:
                     "user_name": self.user_name if self.user_name else None,
                 },
             )
+            # Enter the span context to make it current
+            self.langfuse_span.__enter__()
 
             if self.user_id:
                 logging.info(f"Langfuse: Tracking session for user {self.user_name} ({self.user_id})")
@@ -464,6 +466,8 @@ async def finalize(self, result_payload: dict | None) -> None:
         # Complete Langfuse session span with final results
         if self.langfuse_span and self.langfuse_client and result_payload:
             try:
+                # Exit span context before ending
+                self.langfuse_span.__exit__(None, None, None)
                 self.langfuse_span.end(
                     output=result_payload,
                     metadata={
@@ -527,6 +531,8 @@ async def cleanup_on_error(self, error: Exception) -> None:
         # 1. End Langfuse session span with error if available
         if self.langfuse_span and self.langfuse_client:
             try:
+                # Exit span context before ending
+                self.langfuse_span.__exit__(None, None, None)
                 self.langfuse_span.end(
                     level="ERROR",
                     status_message=str(error)