fix(js): split tool response and other parts in vertexai gemini

[cabljac]

Fixes #452

Only manually tested so far in a couple of specific cases.

Checklist (if applicable):

• Tested (manually, unit tested, etc.)
• Changelog updated
• Docs updated

[MichaelDoyle]

Discussing on Discord, but I am of the opinion this should be fixed further upstream. That could be in the middleware. But I am not 💯 about this interaction.

1. The issue applies to both Google AI and Vertex AI
2. [Prior to this fix] the context appears to "move" from the original user message down into the toolResponse during the tool calling loop. This is definitely not what I would expect. I would expect the original history to be maintained, and a toolResponse simply tacked on. IOW, there should be no need to "move it back".

Reproduced in a trace for Google AI:
https://gist.github.com/MichaelDoyle/a5e4336d92e4353b2cd5237f97200690

[cabljac]

closing in favour of #591