BYOM: Allow custom base URL for self-hosted / internal LLM endpoints

Problem

The current BYOM configuration accepts an API key for a supported provider (Anthropic, OpenAI, Gemini), but does not allow specifying a custom base URL. Enterprise organizations typically route LLM traffic through an internally-hosted proxy or gateway (e.g., LiteLLM) to enforce security controls, model governance, and cost management. In these environments, using a personal or team API key tied to a commercial provider account is not viable. All traffic must go through an internal endpoint on an approved FQDN.

Requested Solution

Add a custom base URL field to the Spacelift AI BYOM configuration, alongside the existing API key field. This would allow Spacelift to direct AI requests to any OpenAI-compatible endpoint (e.g., https://llm.internal.example.com/v1) rather than only to a commercial provider's public API.

This pattern is standard across OpenAI-compatible clients (OPENAI_BASE_URL, openai.base_url in the Python SDK, etc.) and is how tools like LiteLLM, Azure OpenAI, vLLM, and Ollama are accessed.

Use Case

Our organization runs a centrally-managed LiteLLM instance serving internally-approved models via an OpenAI-compatible API. We want Spacelift AI features (plan summaries, resource explanations, policy suggestions) to use this internal endpoint, but the current BYOM flow only accepts a provider API key, with no way to redirect the base URL.

Priority

High. This is a blocker for enterprise customers with internal model governance requirements.