feat(llm): support baseurls for llm providers

[perfectra1n]

Closes #9725

[gemini-code-assist]

Code Review

This pull request introduces the ability to override the default API base URL for LLM providers (Anthropic, OpenAI, and Google Gemini). This is achieved by adding a "Base URL" field to the provider configuration modal in the client, updating the server-side provider interfaces and factories to support an optional baseURL, and passing this value to the respective AI SDKs. New unit tests were also added for each provider to ensure the base URL is correctly handled. I have no feedback to provide.

[perfectra1n]

This would also help for all of the "please also support provider X" instances, as many providers have a "OpenAI compatible" endpoint, so they could just set the OpenAI provider's base URL to whatever they wanted intead of adding a whole other provider...