fix(proxy): preserve Codex OAuth auth token on takeover

[codeasier]

Summary / 概述

修复 Claude Code + Codex OAuth + Claude takeover 场景下，ANTHROPIC_AUTH_TOKEN 被 managed account takeover 强制删除后导致 Claude Code 弹出登录提示的问题。

本 PR 保持现有 ManagedAccount / 非 ManagedAccount 两态分流不变，仅在 managed 分支内对 Codex OAuth 做最小特例处理：

• GitHub Copilot 行为不变：继续删除 ANTHROPIC_AUTH_TOKEN，并写入 ANTHROPIC_API_KEY = PROXY_MANAGED
• Codex OAuth 行为调整：如果 live config 已存在 ANTHROPIC_AUTH_TOKEN，则保留该字段并将值替换为 PROXY_MANAGED
• 非 managed provider 行为不变
• 更新 Codex OAuth takeover 测试断言，保留 Copilot 回归断言

Related Issue / 关联 Issue

Fixes #3784

Screenshots / 截图

• Step 1. 选择一个非codex账号（非ManagedAccount场景），开启takeover及本地路由，查看settings.json

• Step 2. 更换选择为codex provider，保持takeover及本地路由，查看settings.json, 正确注入auth占位符

• Step3. 启动cc

日志如下

Checklist / 检查清单

• pnpm typecheck passes / 通过 TypeScript 类型检查
• pnpm format:check passes / 通过代码格式检查
• cargo clippy passes (if Rust code changed) / 通过 Clippy 检查（如修改了 Rust 代码）
• Updated i18n files if user-facing text changed / 如修改了用户可见文本，已更新国际化文件

Additional verification / 额外验证：

• cargo test managed_account_claude_takeover --manifest-path src-tauri/Cargo.toml
• End-to-end Codex OAuth scenario tested manually

[farion1231]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

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