LiteLLM inline completions don't work with chat-only models like Anthropic

[NiklasPhabian]

Problem

The inline completions (autocomplete) in the LiteLLM provider fail when using chat-only models like Anthropic Claude because the code only uses the prompt/suffix parameters (fill-in-the-middle format):

litellm_resp = litellm.completion(
    model=model_id,
    prompt=prefix,
    suffix=suffix,
    ...
)

This results in errors: litellm.BadRequestError: Anthropic requires at least one non-system message.

Root Cause

• Some models (OpenAI Codex, etc.) support FIM with prompt/suffix parameters
• Chat-only models (Anthropic, most modern LLMs) only support messages format
• The current implementation only tries FIM format

Proposed Solution

Implement a fallback mechanism:

1. Try FIM format first (prompt/suffix) for models that support it
2. If that fails, fall back to messages format with proper prompt construction
3. Strip markdown code blocks from responses (chat models often return formatted code)

This maintains backward compatibility with FIM-capable models while adding support for chat-only models.

Impact

Users can now use Anthropic Claude and other chat-only models for inline completions through the LiteLLM provider.