agent: Support images via @file and the file context picker (#29596)

Release Notes:

- agent: Add support for @mentioning images
- agent: Add support for including images via file context picker

---------

Co-authored-by: Oleksiy Syvokon <oleksiy.syvokon@gmail.com>

crates/agent/src/context.rs

@@ -639,6 +639,7 @@ impl Display for RulesContext {
 
 #[derive(Debug, Clone)]
 pub struct ImageContext {
+    pub project_path: Option<ProjectPath>,
     pub original_image: Arc<gpui::Image>,
     // TODO: handle this elsewhere and remove `ignore-interior-mutability` opt-out in clippy.toml
     // needed due to a false positive of `clippy::mutable_key_type`.