This PR makes it so we pass up the tool results in the `tool_results`
field in the request message to the LLM.
This required reworking how we track non-text content in the context
editor.
We also removed serialization of images in context history, as we were
never deserializing it, and thus it was unneeded.
Release Notes:
- N/A
---------
Co-authored-by: Antonio <antonio@zed.dev>