fix: custom model options in Conversation.chain()

[michaelmdeng]

Normally, llm prompt ... calls use Model.prompt(), which takes model options as Model.prompt(..., **options). However, llm prompt ... calls with tools (or other configurations that require a conversation) use Conversation.chain(), which takes model options as Conversation.chain(..., options: Optional[dict] = None). This causes those calls to fail with invalid options when custom model options are provided.

An easy way to trigger this error is via a custom provider option for OpenRouter models via the llm-openrouter plugin.

$ llm -m openrouter/deepseek/deepseek-chat-v3-0324 -T llm_version "what version of llm is this?" -o provider '{"only": ["deepseek"]}'

Error: Conversation.chain() got an unexpected keyword argument 'provider'

There is no equivalent error when tools are not used:

$ llm -m openrouter/deepseek/deepseek-chat-v3-0324 "what version of llm is this?" -o provider '{"only": ["deepseek"]}'
<response>...

With this fix, we can successfully prompt with the tool call:

$ llm -m openrouter/deepseek/deepseek-chat-v3-0324 -T llm_version "what version of llm is this?" -o provider '{"only": ["deepseek"]}'

The installed version of LLM is 0.26.

[michaelmdeng]

resolved in #1233