Only start tracing worker thread on first span/trace (#804)

Closes #796. Shouldn't start a busy waiting thread if there aren't any
traces.

Test plan
```
import threading
assert threading.active_count() == 1
import agents
assert threading.active_count() == 1
```

src/agents/tracing/processors.py

@@ -188,10 +188,27 @@ class BatchTraceProcessor(TracingProcessor):
         # Track when we next *must* perform a scheduled export
         self._next_export_time = time.time() + self._schedule_delay
 
-        self._worker_thread = threading.Thread(target=self._run, daemon=True)
-        self._worker_thread.start()
+        # We lazily start the background worker thread the first time a span/trace is queued.
+        self._worker_thread: threading.Thread | None = None
+        self._thread_start_lock = threading.Lock()
+
+    def _ensure_thread_started(self) -> None:
+        # Fast path without holding the lock
+        if self._worker_thread and self._worker_thread.is_alive():
+            return
+
+        # Double-checked locking to avoid starting multiple threads
+        with self._thread_start_lock:
+            if self._worker_thread and self._worker_thread.is_alive():
+                return
+
+            self._worker_thread = threading.Thread(target=self._run, daemon=True)
+            self._worker_thread.start()
 
     def on_trace_start(self, trace: Trace) -> None:
+        # Ensure the background worker is running before we enqueue anything.
+        self._ensure_thread_started()
+
         try:
             self._queue.put_nowait(trace)
         except queue.Full:
@@ -206,6 +223,9 @@ class BatchTraceProcessor(TracingProcessor):
         pass
 
     def on_span_end(self, span: Span[Any]) -> None:
+        # Ensure the background worker is running before we enqueue anything.
+        self._ensure_thread_started()
+
         try:
             self._queue.put_nowait(span)
         except queue.Full:
@@ -216,7 +236,13 @@ class BatchTraceProcessor(TracingProcessor):
         Called when the application stops. We signal our thread to stop, then join it.
         """
         self._shutdown_event.set()
-        self._worker_thread.join(timeout=timeout)
+
+        # Only join if we ever started the background thread; otherwise flush synchronously.
+        if self._worker_thread and self._worker_thread.is_alive():
+            self._worker_thread.join(timeout=timeout)
+        else:
+            # No background thread: process any remaining items synchronously.
+            self._export_batches(force=True)
 
     def force_flush(self):
         """