fix: create AsyncOpenAI lazily in trajectory_compressor to avoid closed event loop (#4013)

The AsyncOpenAI client was created once at __init__ and stored as an
instance attribute. process_directory() calls asyncio.run() which creates
and closes a fresh event loop. On a second call, the client's httpx
transport is still bound to the closed loop, raising RuntimeError:
"Event loop is closed" — the same pattern fixed by PR #3398 for the
main agent loop.

Create the client lazily in _get_async_client() so each asyncio.run()
gets a client bound to the current loop.

Co-authored-by: binhnt92 <binhnt.ht.92@gmail.com>

tests/test_trajectory_compressor_async.py

@@ -0,0 +1,115 @@
+"""Tests for trajectory_compressor AsyncOpenAI event loop binding.
+
+The AsyncOpenAI client was created once at __init__ time and stored as an
+instance attribute. When process_directory() calls asyncio.run() — which
+creates and closes a fresh event loop — the client's internal httpx
+transport remains bound to the now-closed loop. A second call to
+process_directory() would fail with "Event loop is closed".
+
+The fix creates the AsyncOpenAI client lazily via _get_async_client() so
+each asyncio.run() gets a client bound to the current loop.
+"""
+
+import types
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+class TestAsyncClientLazyCreation:
+    """trajectory_compressor.py — _get_async_client()"""
+
+    def test_async_client_none_after_init(self):
+        """async_client should be None after __init__ (not eagerly created)."""
+        from trajectory_compressor import TrajectoryCompressor
+
+        comp = TrajectoryCompressor.__new__(TrajectoryCompressor)
+        comp.config = MagicMock()
+        comp.config.base_url = "https://api.example.com/v1"
+        comp.config.api_key_env = "TEST_API_KEY"
+        comp._use_call_llm = False
+        comp.async_client = None
+        comp._async_client_api_key = "test-key"
+
+        assert comp.async_client is None
+
+    def test_get_async_client_creates_new_client(self):
+        """_get_async_client() should create a fresh AsyncOpenAI instance."""
+        from trajectory_compressor import TrajectoryCompressor
+
+        comp = TrajectoryCompressor.__new__(TrajectoryCompressor)
+        comp.config = MagicMock()
+        comp.config.base_url = "https://api.example.com/v1"
+        comp._async_client_api_key = "test-key"
+        comp.async_client = None
+
+        mock_async_openai = MagicMock()
+        with patch("openai.AsyncOpenAI", mock_async_openai):
+            client = comp._get_async_client()
+
+        mock_async_openai.assert_called_once_with(
+            api_key="test-key",
+            base_url="https://api.example.com/v1",
+        )
+        assert comp.async_client is not None
+
+    def test_get_async_client_creates_fresh_each_call(self):
+        """Each call to _get_async_client() creates a NEW client instance,
+        so it binds to the current event loop."""
+        from trajectory_compressor import TrajectoryCompressor
+
+        comp = TrajectoryCompressor.__new__(TrajectoryCompressor)
+        comp.config = MagicMock()
+        comp.config.base_url = "https://api.example.com/v1"
+        comp._async_client_api_key = "test-key"
+        comp.async_client = None
+
+        call_count = 0
+        instances = []
+
+        def mock_constructor(**kwargs):
+            nonlocal call_count
+            call_count += 1
+            instance = MagicMock()
+            instances.append(instance)
+            return instance
+
+        with patch("openai.AsyncOpenAI", side_effect=mock_constructor):
+            client1 = comp._get_async_client()
+            client2 = comp._get_async_client()
+
+        # Should have created two separate instances
+        assert call_count == 2
+        assert instances[0] is not instances[1]
+
+
+class TestSourceLineVerification:
+    """Verify the actual source has the lazy pattern applied."""
+
+    @staticmethod
+    def _read_file() -> str:
+        import os
+        base = os.path.dirname(os.path.dirname(__file__))
+        with open(os.path.join(base, "trajectory_compressor.py")) as f:
+            return f.read()
+
+    def test_no_eager_async_openai_in_init(self):
+        """__init__ should NOT create AsyncOpenAI eagerly."""
+        src = self._read_file()
+        # The old pattern: self.async_client = AsyncOpenAI(...) in _init_summarizer
+        # should not exist — only self.async_client = None
+        lines = src.split("\n")
+        for i, line in enumerate(lines, 1):
+            if "self.async_client = AsyncOpenAI(" in line and "_get_async_client" not in lines[max(0,i-3):i+1]:
+                # Allow it inside _get_async_client method
+                # Check if we're inside _get_async_client by looking at context
+                context = "\n".join(lines[max(0,i-10):i+1])
+                if "_get_async_client" not in context:
+                    pytest.fail(
+                        f"Line {i}: AsyncOpenAI created eagerly outside _get_async_client()"
+                    )
+
+    def test_get_async_client_method_exists(self):
+        """_get_async_client method should exist."""
+        src = self._read_file()
+        assert "def _get_async_client(self)" in src