Fixes context middleware to append to user message only.

[mbleigh]

Applies the fix from #542 to the context augmentation middleware.

Checklist (if applicable):

• Tested (manually, unit tested, etc.)

[MichaelDoyle]

Thanks! This change itself looks great.

On a bigger picture level, I am still curious as to whether or not the middleware should be reapplied during a tool-calling loop, or if we can/should maintain the original request that went to the LLM and simple append to that. I am not so familiar with how this is al plumbed together.