Python: fix(devui): page list_items past checkpoint items - #9126

Motivation & Context

InMemoryConversationStore.list_items adds workflow checkpoints to the list as plain dicts, but the after cursor loop read item.id, so paging a conversation that has a checkpoint raised AttributeError: 'dict' object has no attribute 'id' (HTTP 500 from GET /v1/conversations/{id}/items). With order="desc" that is every after request, since checkpoints come first.

Description & Review Guide

• What are the major changes? The cursor loop reads the id from the dict for checkpoint items and from .id for the rest.
• What is the impact of these changes? Paging works across checkpoint items. Results are unchanged for conversations without checkpoints.
• What do you want reviewers to focus on? Nothing special; it is the same dict/model split the loop that builds the list already uses.

Tests: test_list_items_pagination_with_checkpoint_items[asc|desc] in tests/devui/test_conversations.py pages with limit=1 over two messages and two checkpoints and checks the ids match the unpaged list; both fail on main. devui tests pass (excluding test_ui_memory_regression.py, which needs a browser). ruff check and format clean.

Related Issue

Fixes #9125

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

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