Sometimes the contents in the agent's thread are direct file links.
Codex ACP almost every time uses absolute file paths instead of
file://path/to/file. This is resulting in an error described in the
#49978 .


Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #49978

Release Notes:

- Fixed absolute path mentions in `acp_thread` so direct file references
(including `:line` and `#Lline` formats) now open correctly instead of
resolving to invalid/empty files.

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Co-authored-by: Eric Holk <eric@zed.dev>