Add LiteLLM as a new OpenAI-compatible proxy provider:
- Add onboarding flow with API key, base URL, and model selection
- Fetch available models from LiteLLM /v1/models endpoint
- Auto-detect context window from /model/info endpoint
- Set supportsStore: false to avoid "Extra inputs are not permitted" errors
with providers that don't support the OpenAI Responses API store parameter
- Preserve compat settings through model resolution pipeline
- Add provider documentation
Closes#2639Closes#2305
Co-Authored-By: Claude <noreply@anthropic.com>
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