Add tool support for DeepSeek

[thlstsul]

deepseek function call api has been released and it is same as openai.

Release Notes:

• N/A

[bennetbo]

Seems like deepseek-reasoner also supports function calling, as stated here: https://api-docs.deepseek.com/guides/reasoning_model

Tested it locally and seems to work for both models.

[medz]

To simplify maintenance, I think it would be better to reuse OpenAI-compatible API LLMs with OpenAI logic. However, the context length needs to be configured separately.

For example, DeepSeek does not have a special API, but is published using the OpenAI API-compatible API. This should help Zed's large model support maintenance.