[codex] 支持自定义平台 API Key

[KilimiaoSix]

Summary

• Add optional custom platform API key support when creating platform keys.
• Validate custom keys for empty input, whitespace/control characters, length, and duplicates.
• Wire the new customKey parameter through the frontend client, Tauri command, RPC dispatch, and service layer.
• Update ccswitch/Codex CLI docs and cover the behavior with RPC and Playwright tests.

Closes #190

Validation

• cargo test -p codexmanager-service --test rpc rpc_apikey_create_accepts_custom_key_and_rejects_duplicate
• pnpm exec playwright test tests/api-keys-model-prefix.spec.ts
• cargo check --manifest-path apps/src-tauri/Cargo.toml
• pnpm run build:desktop
• git diff --check