fix: byok compatibility

[wjiayis]

Problem Description

Prod breaks when user provides their own API key.

Cause of Problem

This is because endpoint, API key and model name are hardcoded to be OpenRouter compatible in some methods.

Solution

This PR fixes this breakage by making the definition of endpoint, API key and model name conditional on whether user specifies their own API key.

Notes to Reviewers

Pls help to test this PR's compatibility with OpenRouter API. I only tested it with OpenAI API as I only have OpenAI API key.

[wjiayis]

Sure, I'll get it done tmr 25/01

[wjiayis]

@4ndrelim Updated my fix based on your suggested changes!

The deeper root cause of this bug is that in getDefaultParamsV2, the input modelSlug is by default prefixed with openai/. While the logic in the method implements a substring match, the final parameter passed to openaiv3.ChatCompletionNewParams is still the original modelSlug. I stripped the prefix if a custom model is passed.

Edit: This is the same issue as #99. Either this PR or #100 (which debugs #99) can be used to solve the problem. On hindsight, #100 is probably a more native fix.

[wjiayis]

Closing this PR as it solves the same purpose as #100, with #100 being a more native fix.