Feat: support custom provider in codex


When I run:

```
bex-security scan . --agent codex --provider cliproxyapi --model agnes/agnes-3.0-flash --effort high
```

I got this error:
```
codex-security: Error: invalid value for --provider: Invalid option: expected one of "openai"|"openrouter"|"fireworks"|"amazon-bedrock"|"zai"|"kimi"
See below for usage.
```

Is it ok to support custom provider for codex cli as well? Thanks