chore: import upstream snapshot with attribution

tests/realtime/test_playback_tracker_manual_unit.py

@@ -0,0 +1,23 @@
+from agents.realtime.model import RealtimePlaybackTracker
+
+
+def test_playback_tracker_on_play_bytes_and_state():
+    tr = RealtimePlaybackTracker()
+    tr.set_audio_format("pcm16")  # PCM path
+
+    # 48k bytes -> (48000 / (24000 * 2)) * 1000 = 1_000ms
+    tr.on_play_bytes("item1", 0, b"x" * 48000)
+    st = tr.get_state()
+    assert st["current_item_id"] == "item1"
+    assert st["elapsed_ms"] and abs(st["elapsed_ms"] - 1_000.0) < 1e-6
+
+    # Subsequent play on same item accumulates
+    tr.on_play_ms("item1", 0, 500.0)
+    st2 = tr.get_state()
+    assert st2["elapsed_ms"] and abs(st2["elapsed_ms"] - 1_500.0) < 1e-6
+
+    # Interruption clears state
+    tr.on_interrupted()
+    st3 = tr.get_state()
+    assert st3["current_item_id"] is None
+    assert st3["elapsed_ms"] is None