test(gateway): add regression test for /retry response fix

Adds two tests for _handle_retry_command: verifies /retry returns the
agent response (not None), and verifies graceful handling when no
previous message exists.

Cherry-picked from PR #731 by teyrebaz33. Regression coverage for
the fix merged in PR #441.

Co-authored-by: teyrebaz33 <teyrebaz33@users.noreply.github.com>

tests/gateway/test_retry_response.py

@@ -0,0 +1,60 @@
+"""Regression test: /retry must return the agent response, not None.
+
+Before the fix in PR #441, _handle_retry_command() called
+_handle_message(retry_event) but discarded its return value with `return None`,
+so users never received the final response.
+"""
+import pytest
+from unittest.mock import AsyncMock, MagicMock
+from gateway.run import GatewayRunner
+from gateway.platforms.base import MessageEvent, MessageType
+
+
+@pytest.fixture
+def gateway(tmp_path):
+    config = MagicMock()
+    config.sessions_dir = tmp_path
+    config.max_context_messages = 20
+    gw = GatewayRunner.__new__(GatewayRunner)
+    gw.config = config
+    gw.session_store = MagicMock()
+    return gw
+
+
+@pytest.mark.asyncio
+async def test_retry_returns_response_not_none(gateway):
+    """_handle_retry_command must return the inner handler response, not None."""
+    gateway.session_store.get_or_create_session.return_value = MagicMock(
+        session_id="test-session"
+    )
+    gateway.session_store.load_transcript.return_value = [
+        {"role": "user", "content": "Hello Hermes"},
+        {"role": "assistant", "content": "Hi there!"},
+    ]
+    gateway.session_store.rewrite_transcript = MagicMock()
+    expected_response = "Hi there! (retried)"
+    gateway._handle_message = AsyncMock(return_value=expected_response)
+    event = MessageEvent(
+        text="/retry",
+        message_type=MessageType.TEXT,
+        source=MagicMock(),
+    )
+    result = await gateway._handle_retry_command(event)
+    assert result is not None, "/retry must not return None"
+    assert result == expected_response
+
+
+@pytest.mark.asyncio
+async def test_retry_no_previous_message(gateway):
+    """If there is no previous user message, return early with a message."""
+    gateway.session_store.get_or_create_session.return_value = MagicMock(
+        session_id="test-session"
+    )
+    gateway.session_store.load_transcript.return_value = []
+    event = MessageEvent(
+        text="/retry",
+        message_type=MessageType.TEXT,
+        source=MagicMock(),
+    )
+    result = await gateway._handle_retry_command(event)
+    assert result == "No previous message to retry."