Create a function to custom API Endpoint by users

[Sigechaishijie]

Problem

Users are not able to custom API Endpoint by themselves

Proposed solution

Just add a function to edit API Endpoint

Use case

Users are not able to custom API Endpoint by themselves, so that they must use DeepSeek's official server, which can't be used to dispose private things

Alternatives considered

It isn't suitable for this request

Impact

Not very often, but some users use it very often. This function can help me save my cost

[Hmbown]

Thanks for the request. PR #1967 is the current candidate for the narrow DeepSeek base_url configuration path.

I am keeping the broader custom-endpoint work tied to v0.8.46 because endpoint configuration needs to line up with #1874 and #1978: base URL, path shape, provider parser choice, reasoning/cache parsing, and readable failure messages should not diverge across OpenAI-compatible providers.