Add GitLab Duo auth and executor support

[LuxVTZ]

Summary

Add GitLab Duo support that behaves much closer to the built-in codex provider:

• OAuth login for GitLab Duo
• personal access token login for GitLab Duo
• refresh of direct_access metadata and managed credentials
• dynamic model discovery from GitLab metadata instead of a hardcoded model list
• native GitLab AI gateway routing for Anthropic-managed Duo models
• native GitLab AI gateway routing for OpenAI/Codex-managed Duo models
• handler-level smoke coverage for Claude-compatible and OpenAI-compatible downstream APIs
• user-facing GitLab Duo guides in English and Chinese

Details

GitLab Duo now routes by provider-aware transport instead of relying only on the GitLab text wrapper APIs:

• Anthropic-managed models delegate through the GitLab AI gateway Anthropic proxy and the existing ClaudeExecutor
• OpenAI/Codex-managed models delegate through the GitLab AI gateway OpenAI proxy and the existing CodexExecutor
• /v1/messages, /v1/chat/completions, and /v1/responses are covered by smoke tests

Docs added:

• docs/gitlab-duo.md
• docs/gitlab-duo_CN.md

Testing

• go test ./...

[luispater]

Since the creation of the CLIProxyAPI project, our first rule has been "No third-party resellers."

So, we created a new repository for third-party reseller providers.

You can open a PR to the new repository: CLIProxyAPIPlus

[gemini-code-assist]

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[LuxVTZ]

Moving this work to CLIProxyAPIPlus per maintainer guidance. New PR: router-for-me#428