provider: fix codex native provider name

[duoyuli]

No description provided.

[Copilot]

Pull request overview

Ensures Codex proxy provider profiles written by arc-kit always use Codex’s expected native provider identifier (name = "OpenAI"), while still selecting the desired profile via the arc provider profile name.

Changes:

• Update Codex config writer to persist a fixed native provider name value ("OpenAI") for proxy profiles.
• Adjust/add tests to validate the written Codex TOML and auth JSON behavior (including a CLI black-box test).
• Document the behavior in README and the user guides (EN + zh-CN).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Documents that Codex proxy providers keep native name fixed to OpenAI.
docs/user/guide.md	Adds the same rule to the English user guide.
docs/user/guide.zh-CN.md	Adds the same rule to the Chinese user guide.
arc-core/src/provider/codex.rs	Writes Codex proxy provider name as a fixed "OpenAI" constant.
arc-core/tests/provider_tests.rs	Updates the Codex config-writing test expectation to "OpenAI".
arc-cli/tests/provider_codex_config.rs	Adds an integration test ensuring CLI provider use writes native name as "OpenAI" and sets OPENAI_API_KEY.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.