fix: add resolve source path

[Nexisato]

Description

• 多机器挂载时 save 逻辑的 path 解析问题

Close: #1691

[gemini-code-assist]

Code Review

This pull request introduces a path resolution helper _resolve_save_source in sender.py to handle cross-machine synchronization scenarios where the original absolute source_path is unreadable. It implements fallback logic to look up files within the run's files directory, and updates upload_save to use this resolved path. Comprehensive unit tests are also added to verify this fallback behavior. The reviewer pointed out a potential cross-platform issue where Windows-style backslashes in save.source_path or save.name would fail to resolve correctly on POSIX systems, and suggested using PureWindowsPath to normalize the paths.

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