fix(acp): follow-up — named-const page size, alias kwarg, tests

- Replace kwargs.get('limit', 50) with module-level _LIST_SESSIONS_PAGE_SIZE
  constant. ListSessionsRequest schema has no 'limit' field, so the kwarg
  path was dead. Constant is the single source of truth for the page cap.
- Use next_cursor= (field name) instead of nextCursor= (alias). Both work
  under the schema's populate_by_name config, but using the declared
  Python field name is the consistent style in this file.
- Add docstring explaining cwd pass-through and cursor semantics.
- Add 4 tests: first-page with next_cursor, single-page no next_cursor,
  cursor resumes after match, unknown cursor returns empty page.

tests/acp/test_server.py

@@ -270,6 +270,57 @@ class TestListAndFork:
 
         mock_list.assert_called_once_with(cwd="/mnt/e/Projects/AI/browser-link-3")
 
+    @pytest.mark.asyncio
+    async def test_list_sessions_pagination_first_page(self, agent):
+        from acp_adapter import server as acp_server
+
+        infos = [
+            {"session_id": f"s{i}", "cwd": "/tmp", "title": None, "updated_at": 0.0}
+            for i in range(acp_server._LIST_SESSIONS_PAGE_SIZE + 5)
+        ]
+        with patch.object(agent.session_manager, "list_sessions", return_value=infos):
+            resp = await agent.list_sessions()
+
+        assert len(resp.sessions) == acp_server._LIST_SESSIONS_PAGE_SIZE
+        assert resp.next_cursor == resp.sessions[-1].session_id
+
+    @pytest.mark.asyncio
+    async def test_list_sessions_pagination_no_more(self, agent):
+        infos = [
+            {"session_id": f"s{i}", "cwd": "/tmp", "title": None, "updated_at": 0.0}
+            for i in range(3)
+        ]
+        with patch.object(agent.session_manager, "list_sessions", return_value=infos):
+            resp = await agent.list_sessions()
+
+        assert len(resp.sessions) == 3
+        assert resp.next_cursor is None
+
+    @pytest.mark.asyncio
+    async def test_list_sessions_cursor_resumes_after_match(self, agent):
+        infos = [
+            {"session_id": "s1", "cwd": "/tmp", "title": None, "updated_at": 0.0},
+            {"session_id": "s2", "cwd": "/tmp", "title": None, "updated_at": 0.0},
+            {"session_id": "s3", "cwd": "/tmp", "title": None, "updated_at": 0.0},
+        ]
+        with patch.object(agent.session_manager, "list_sessions", return_value=infos):
+            resp = await agent.list_sessions(cursor="s1")
+
+        assert [s.session_id for s in resp.sessions] == ["s2", "s3"]
+        assert resp.next_cursor is None
+
+    @pytest.mark.asyncio
+    async def test_list_sessions_unknown_cursor_returns_empty(self, agent):
+        infos = [
+            {"session_id": "s1", "cwd": "/tmp", "title": None, "updated_at": 0.0},
+            {"session_id": "s2", "cwd": "/tmp", "title": None, "updated_at": 0.0},
+        ]
+        with patch.object(agent.session_manager, "list_sessions", return_value=infos):
+            resp = await agent.list_sessions(cursor="does-not-exist")
+
+        assert resp.sessions == []
+        assert resp.next_cursor is None
+
 # ---------------------------------------------------------------------------
 # session configuration / model routing
 # ---------------------------------------------------------------------------