chore: improve rollout session init errors

[jobchong]

Title: Improve rollout session initialization error messages

Issue: #7283

What: add targeted mapping for rollout/session initialization errors so users get actionable messages when Codex cannot access session files.

Why: session creation previously returned a generic internal error, hiding permissions/FS issues and making support harder.

How:

• Added rollout::error::map_session_init_error to translate the more common io::Error kinds into user-facing hints (permission, missing dir, file blocking, corruption). Others are passed through directly with CodexErr::Fatal.
• Reused the mapper in Codex session creation to preserve root causes instead of returning InternalAgentDied.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[jobchong]

I have read the CLA Document and I hereby sign the CLA

[etraut-openai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Nice work!

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[etraut-openai]

Thanks for the contribution! Overall, this is looking good. I left one suggested change.

[jobchong]

I've removed the tests!