fix(codex): prefer npm binary over WindowsApps

[somewherelostt]

Fixes #2582

Summary

• Prefer the npm-installed Codex shim/native binary on Windows before probing PATH.
• Skip the packaged WindowsApps OpenAI Codex executable when it appears in PATH, because it can shadow the authenticated npm install and return false auth failures.
• Add a Windows regression test for the npm-vs-WindowsApps resolver ordering.

Test

• cd backend && go test ./internal/adapters/agent/codex

[somewherelostt]

fixed