Fixes#3939Fixes#2803
## Summary
- convert Windows clipboard file paths into their `/mnt/<drive>`
equivalents when running inside WSL so pasted images resolve correctly
- add WSL detection helpers and share them with unit tests to cover both
native Windows and WSL clipboard normalization cases
- improve the test suite by exercising Windows path handling plus a
dedicated WSL conversion scenario and keeping the code path guarded by
targeted cfgs
## Testing
- just fmt
- cargo test -p codex-tui
- cargo clippy -p codex-tui --tests
- just fix -p codex-tui
## Screenshots
_Codex TUI screenshot:_
<img width="1880" height="848" alt="describe this copied image"
src="https://github.com/user-attachments/assets/c620d43c-f45c-451e-8893-e56ae85a5eea"
/>
_GitHub docs directory screenshot:_
<img width="1064" height="478" alt="image-copied"
src="https://github.com/user-attachments/assets/eb5eef6c-eb43-45a0-8bfe-25c35bcae753"
/>
Co-authored-by: Eric Traut <etraut@openai.com>
