Fix Codex ChatGPT OAuth auth handling

[etailup]

Summary

• Treat Codex OAuth entries as full Codex auth.json payloads instead of single API-key-style tokens
• Write Codex ChatGPT OAuth payloads to ~/.codex/auth.json inside Codex containers
• Update the OpenAI OAuth modal and docs to ask for Codex auth.json contents

Verification

• Validated Codex auth script writes auth.json for a ChatGPT OAuth payload
• Validated API-key auth JSON is rejected in Codex OAuth mode
• Ran bash -n docker/coding-agent/scripts/agents/codex-cli/auth.sh
• Ran git diff --check
• Ran npm run build
• Ran npm test