Support both tool call and message text handling in response

[Finndersen]

At the moment it appears that only only Tool calls OR message text processing is supported, not both at the same time.

I'm not sure if all LLMs support providing both, but it appears some do:
OpenAI example
Anthropic documentation appears to indicate it is possible

There are cases where a LLM may perform a tool call as a final response (doesn't need to see the results), along with an associated relevant message. Enabling handling of both would avoid an additional request/response cycle to the LLM.

I think this is somewhat related to this issue: #127, and this PR: #142, however slightly different since I believe those two are about whether a tool call can end a "conversation", which would also be a necessary capability to resolve this issue, but would also involve additional capability to handle both the tool call and then return the message text as the "final message" content.