fix(frontend): paginate run history messages

[LittleChenLiya]

问题原因

聊天历史加载按 run 逐个请求消息，但前端每个 run 只读取默认第一页。后端返回的是该 run 最新一页消息，并通过 has_more 表示还有更早消息；前端没有识别这个字段，也没有带 before_seq 继续加载同一个 run，导致长对话加载更多时会遗漏同一 run 内更早的消息，后续再拼接其他 run 时表现为历史缺失或顺序错位。

修改内容

• 为 run 历史加载增加同一 run 内的 before_seq 游标。
• 正确识别后端返回的 has_more 字段，只有当前 run 全部分页加载完成后才切换到更早的 run。
• 补充单元测试覆盖 has_more 识别和下一页 cursor 计算。

关联 issue

关联 #3052

[Copilot]

Pull request overview

This PR fixes missing/misordered chat history in the frontend by correctly paginating messages within a single run. It aligns the frontend with the backend run-messages API contract (has_more + before_seq) so “load more” continues paging the same run until exhausted before moving to older runs.

Changes:

• Add run-internal pagination support via before_seq cursor tracking per run_id.
• Correctly interpret backend pagination flag (has_more), while keeping compatibility with the previous hasMore.
• Add unit tests for has_more handling and next-page cursor computation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
frontend/src/core/threads/hooks.ts	Implements has_more detection and per-run before_seq cursor tracking to paginate run messages correctly.
frontend/src/core/threads/types.ts	Extends RunMessage with optional seq used for cursor pagination.
frontend/tests/unit/core/threads/message-merge.test.ts	Adds unit tests covering has_more compatibility and cursor (seq) selection logic.