fix(tui): handle WSL clipboard image paths

[manoelcalixto]

Fixes #3939
Fixes #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:

GitHub docs directory screenshot:

[Hardanish-Singh]

✅