Add Evolink (evolink.ai) as an OpenAI-compatible model provider

### Feature request Add **Evolink** (https://evolink.ai) as a model provider in `camel.models`. Evolink is a unified gateway that exposes an **OpenAI-compatible** chat completions API at `https://direct.evolink.ai/v1`, giving access to many models (e.g. `gpt-5.x`, `gemini-2.5/3.x`, `deepseek-v4`, `doubao-seed-2.0`, `minimax-m2.5`, `evolink-auto`). ### Proposed implementation Follow the existing thin-wrapper pattern (`AihubMixModel`): a new `EvolinkModel(OpenAICompatibleModel)`, registered in `ModelFactory`, with a `ModelPlatformType.EVOLINK` enum entry. Auth via `EVOLINK_API_KEY`. I have an implementation ready and will open a PR linked to this issue.