fix(frontend): avoid duplicate optimistic user message

[LittleChenLiya]

问题原因

首轮发送后，服务端返回的 human 消息和本地 optimistic human 消息会在 effect 清理 optimistic 状态前短暂同时进入消息合并结果，导致界面上看起来用户 prompt 重复出现。

修改内容

在消息合并前根据 human 消息数量变化过滤已经由服务端确认的 optimistic human 消息，并补充首轮、后续轮次和上传 optimistic 组合的单元测试。

关联 issue

关联 #2995

[Copilot]

Pull request overview

This PR fixes a frontend chat UI race where an optimistic user message and the server-confirmed human message could briefly render together before optimistic state cleanup runs.

Changes:

• Adds getVisibleOptimisticMessages to suppress optimistic human messages as soon as the server human count advances.
• Uses the filtered optimistic messages when merging history, live stream messages, and optimistic UI messages.
• Adds unit coverage for first-turn, later-turn, and upload optimistic-message cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
frontend/src/core/threads/hooks.ts	Filters optimistic messages before merge to avoid transient duplicate user prompts.
frontend/tests/unit/core/threads/message-merge.test.ts	Adds tests for optimistic-message visibility and merge behavior.