Support tools role in messages in the pipeline

[jhrozek]

The open interpreter support in PR #820 added handling of role: tool messages scoped to open interpreter. This was the right thing to scope them to the tool as we wanted to get the tool support in, but we should support that message role in general in our pipeline.

more todo items would probably come out when we do the work but for now I think we should do:

• when messages with that role are processed (e.g. get_last_user_message_block) do we take the tools role into account
• make sure we don't touch those in pipeline to avoid breaking the tools

We can add a bunch of tests just against the openai/anthropic api to make sure we follow their APIs, no need to tie this to a specific client.

[yrobla]

It would also be worth investigating how this role is being used in other tools (if that's used at all) and how to better support it