test(gateway): cover /compress summary-failure warning path

PR #16333 added a warning to the manual /compress reply when the
auxiliary summariser fails and the static fallback placeholder is
used, but only the gateway-hygiene path had a test
(test_session_hygiene_warns_user_when_summary_generation_fails).
The /compress branch in _handle_compress_command was uncovered.

New test test_compress_command_appends_warning_when_summary_generation_fails
mocks the compressor's _last_summary_fallback_used /
_last_summary_dropped_count / _last_summary_error fields and
verifies the /compress reply contains the ⚠️ marker, the underlying
error string, the dropped message count, and the 'historical
message(s) were removed' wording — i.e. the same contract the
hygiene-path test enforces.

tests/gateway/test_compress_command.py

@@ -123,3 +123,61 @@ async def test_compress_command_explains_when_token_estimate_rises():
     assert "denser summaries" in result
     agent_instance.shutdown_memory_provider.assert_called_once()
     agent_instance.close.assert_called_once()
+
+
+@pytest.mark.asyncio
+async def test_compress_command_appends_warning_when_summary_generation_fails():
+    """When the auxiliary summariser fails and the compressor inserts a static
+    fallback placeholder, /compress must append a visible ⚠️ warning to its
+    reply. Otherwise the failure is silently logged and the user has no idea
+    earlier context is unrecoverable."""
+    history = _make_history()
+    # Compressed shape is irrelevant for this test — we only care that the
+    # warning surfaces. Drop one message so the headline is non-noop.
+    compressed = [
+        history[0],
+        {"role": "assistant", "content": "[fallback placeholder]"},
+        history[-1],
+    ]
+    runner = _make_runner(history)
+    agent_instance = MagicMock()
+    agent_instance.shutdown_memory_provider = MagicMock()
+    agent_instance.close = MagicMock()
+    agent_instance.context_compressor.has_content_to_compress.return_value = True
+    # Simulate summary-generation failure: fallback flag set, dropped count
+    # populated, error string captured.
+    agent_instance.context_compressor._last_summary_fallback_used = True
+    agent_instance.context_compressor._last_summary_dropped_count = 7
+    agent_instance.context_compressor._last_summary_error = (
+        "404 model not found: gemini-3-flash-preview"
+    )
+    agent_instance.session_id = "sess-1"
+    agent_instance._compress_context.return_value = (compressed, "")
+
+    def _estimate(messages):
+        if messages == history:
+            return 100
+        if messages == compressed:
+            return 60
+        raise AssertionError(f"unexpected transcript: {messages!r}")
+
+    with (
+        patch("gateway.run._resolve_runtime_agent_kwargs", return_value={"api_key": "***"}),
+        patch("gateway.run._resolve_gateway_model", return_value="test-model"),
+        patch("run_agent.AIAgent", return_value=agent_instance),
+        patch("agent.model_metadata.estimate_messages_tokens_rough", side_effect=_estimate),
+    ):
+        result = await runner._handle_compress_command(_make_event())
+
+    # The compress reply itself still goes through (the transcript was rewritten).
+    assert "Compressed:" in result
+    # ...but a clearly-marked warning must be appended.
+    assert "⚠️" in result
+    assert "Summary generation failed" in result
+    # Underlying error must surface so users can fix their config.
+    assert "404 model not found" in result
+    # Dropped count must be visible — silently losing N messages is the bug.
+    assert "7" in result
+    assert "historical message(s) were removed" in result
+    agent_instance.shutdown_memory_provider.assert_called_once()
+    agent_instance.close.assert_called_once()