assistant: Pass up tool results in LLM request messages (#17656)

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>

crates/paths/src/paths.rs

@@ -170,12 +170,6 @@ pub fn contexts_dir() -> &'static PathBuf {
     })
 }
 
-/// Returns the path within the contexts directory where images from contexts are stored.
-pub fn context_images_dir() -> &'static PathBuf {
-    static CONTEXT_IMAGES_DIR: OnceLock<PathBuf> = OnceLock::new();
-    CONTEXT_IMAGES_DIR.get_or_init(|| contexts_dir().join("images"))
-}
-
 /// Returns the path to the contexts directory.
 ///
 /// This is where the prompts for use with the Assistant are stored.