test(agent): regression for token-only compression progress (#39550, #23767)

Adds test_413_retries_on_token_only_compression: same message count but
materially fewer tokens after compaction must count as progress and retry,
not abort. Fails on main without the salvaged fix, passes with it.

tests/run_agent/test_413_compression.py

@@ -440,6 +440,48 @@ class TestHTTP413Compression:
         assert result.get("partial") is True
         assert "413" in result["error"]
 
+    def test_413_retries_on_token_only_compression(self, agent):
+        """Same message COUNT but fewer TOKENS must count as progress and retry.
+
+        Regression for #39550/#23767: tool-result pruning / in-place
+        summarization can shrink request size without dropping the message
+        count. The old gate (len(messages) < original_len) treated that as
+        'cannot compress further' and aborted; the fix re-estimates tokens and
+        retries when they drop materially.
+        """
+        err_413 = _make_413_error()
+        ok_resp = _mock_response(content="OK after token-only compaction", finish_reason="stop")
+        agent.client.chat.completions.create.side_effect = [err_413, ok_resp]
+
+        # 3 large messages in, 3 much smaller messages out (same count, far
+        # fewer tokens) — exactly the token-only-progress case.
+        prefill = [
+            {"role": "user", "content": "x" * 4000},
+            {"role": "assistant", "content": "y" * 4000},
+            {"role": "user", "content": "z" * 4000},
+        ]
+
+        with (
+            patch.object(agent, "_compress_context") as mock_compress,
+            patch.object(agent, "_persist_session"),
+            patch.object(agent, "_save_trajectory"),
+            patch.object(agent, "_cleanup_task_resources"),
+        ):
+            # Same message count (3) but ~10x smaller content → token drop.
+            mock_compress.return_value = (
+                [
+                    {"role": "user", "content": "x" * 300},
+                    {"role": "assistant", "content": "y" * 300},
+                    {"role": "user", "content": "z" * 300},
+                ],
+                "compressed prompt",
+            )
+            result = agent.run_conversation("hello", conversation_history=prefill)
+
+        mock_compress.assert_called_once()
+        assert result["completed"] is True
+        assert result["final_response"] == "OK after token-only compaction"
+
 
 class TestPreflightCompression:
     """Preflight compression should compress history before the first API call."""