Create a custom OpenAI provider to use multiple models

[srdas]

Description

• Fixes Add support for embedding models served through an OpenAI API #1240.
• Fixes Bug: Model field inputs are coupled in Settings UI #1261
• Related to PR Simplifying the OpenAI provider to use multiple model providers #1248 (closed to open this extended one)

The OpenAI model interface has been widely adopted by many model providers (DeepSeek, vLLM, etc.) and this PR enables accessing these models using the OpenAI provider. Current OpenAI models are also accessible via the same interface.

This PR also updates related documentation on the use of these models that work via the OpenAI provider.

These updates work for selecting chat and embeddings models. Chat models are tested to work with models from OpenAI, DeepSeek, and models hosted by vLLM. Embedding models are tested for OpenAI models. DeepSeek does not have an API for embedding models, and OpenRouter also does not support as yet any embedding models.

Finally, corrected the coupled fields problem in the AI Settings page.

Demo of new features

See the new usage of models and the required settings shown below, note the new "OpenAI::general interface":

For any OpenAI model:

For DeepSeek models:

For models deployed with vLLM:

Embedding Models

First, tested to make sure that the OpenAI models are working as intended with no changes to the code:

Second, modified check that the interface takes any OpenAI embedding model as an input and test that it works with OpenAI models as before:

Fixed coupled model field inputs

We can see that the fields are not coupled any more as shown below: