Issue Description
Quest模型下，检查某个过程中的Diff都会指向最终的Diff，而不是当前修改操作的Diff，这样不知道Agent在过程中某次操作所对应修改的Diff是那几行，只能看到一个文件的总Diff，可以参考一下VSCode Copilot的相关实现。
Under the Quest model, checking the Diff in a certain process will point to the final Diff, not the Diff of the current modification operation. In this way, we do not know which lines of Diff are modified by the Agent in a certain operation during the process. We can only see the total Diff of a file. You can refer to the related implementation of VSCode Copilot.
Steps to Reproduce
Quest模式下，让Agent多次修改一个文件时。
In Quest mode, when letting the Agent modify a file multiple times.
Expected Behavior
点击过程中的某次Diff修改，可以只看这次修改的Diff（不包含当前文件的其他Diff内容）。
Click on a certain Diff modification in the process to view only the modified Diff (excluding other Diff contents of the current file).
Actual Behavior
点击过程中的某次Diff修改，只能看到当前文件的所有修改Diff。
Click on a certain Diff modification in the process, and you can only see all modified Diffs of the current file.
Operating System
Windows10
Current Qoder Version (Menu → About Qoder → Copy)
Version: 0.14.2
VSCode Version: 1.106.3 (user setup)
Commit: a112e9bbdf1687e5a9c730304761b20202c2a352
Date: 2026-04-24T12:37:05.793Z
Electron: 37.7.0
Chromium: 138.0.7204.251
Node.js: 22.20.0
V8: 13.8.258.32-electron.0
OS: Windows_NT x64 10.0.19045