fix(gateway): normalize MSYS paths in validate_media_delivery_path

[nankingjing]

Summary

• Fixes [Bug]: Windows MSYS/Git Bash paths rejected by validate_media_delivery_path(), blocking MEDIA directive sends #47767
• MEDIA:/c/Users/... paths from Git Bash were rejected by validate_media_delivery_path() because pathlib on Windows does not treat MSYS paths as absolute.
• Reuse the existing _msys_to_windows_path() helper from tools.environments.local (same normalization already used for terminal cwd resolution).

Test plan

• pytest tests/gateway/test_platform_base.py::TestMediaDeliveryDefaultMode::test_accepts_msys_path_after_windows_normalization

[alt-glitch]

This was generated by AI during triage.

Duplicate of #47821 — both fix #47767 by normalizing MSYS/Git-Bash paths via _msys_to_windows_path() inside validate_media_delivery_path() (gateway/platforms/base.py). Same file, same function, same mechanism. #47821 is the earlier canonical PR.

[nankingjing]

Closing in favor of the earlier canonical fix path that the maintainer review identified in the comment thread.

Per the triage comment on this PR:

• fix(tools): honor plugin web providers in extract/search dispatch #57704: competing approach in the same plugin-web-provider cluster (fix(web_tools): delegate _is_backend_available to provider registry for plugin backends #31829 / fix(tools): honor available plugin extract/search backends in web selection #35327)
• fix(agent): honor custom_providers per-model max_tokens at startup #57548: duplicate of fix(agent): honor custom provider max_tokens #28154 (earliest still-open, broader superset)
• fix(kanban): terminate workers when tasks are blocked or archived #57713: duplicate of fix(kanban): terminate live workers on manual exits from running #31365 (broader superset covering done/scheduled manual exits too)
• fix(state): avoid double-encoding reasoning columns on fork round-trip #57536: duplicate of fix(state,cli,tui-gateway): keep reasoning fields intact across forks and branches #57248 (earlier canonical version with same write-path-only fix)
• fix(gateway): normalize MSYS paths in validate_media_delivery_path #57696: duplicate of fix(gateway): convert MSYS/Git Bash paths in validate_media_delivery_path (#47767) #47821 (earlier canonical MSYS path fix)

Keeping the narrower cluster open does not help the maintainer pick a canonical fix; closing these reduces the supersede noise so the maintainer can land the right one. Thanks to the alt-glitch triage for the cross-references.