assistant2: Suggest recent files and threads as context (#22959)

The context picker will now display up to 6 recent files/threads to add
as a context:

<img
src="https://github.com/user-attachments/assets/80c87bf9-70ad-4e81-ba24-7a624378b991"
width=400>



Note: We decided to use a `ContextMenu` instead of `Picker` for the
initial one since the latter didn't quite fit the design for the
"Recent" section.

Release Notes:

- N/A

---------

Co-authored-by: Danilo <danilo@zed.dev>
Co-authored-by: Piotr <piotr@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>

crates/assistant2/src/context_store.rs

@@ -2,7 +2,7 @@ use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
 use anyhow::{anyhow, bail, Result};
-use collections::{BTreeMap, HashMap};
+use collections::{BTreeMap, HashMap, HashSet};
 use futures::{self, future, Future, FutureExt};
 use gpui::{AppContext, AsyncAppContext, Model, ModelContext, SharedString, Task, WeakView};
 use language::Buffer;
@@ -372,6 +372,23 @@ impl ContextStore {
             }
         }
     }
+
+    pub fn file_paths(&self, cx: &AppContext) -> HashSet<PathBuf> {
+        self.context
+            .iter()
+            .filter_map(|context| match context {
+                Context::File(file) => {
+                    let buffer = file.context_buffer.buffer.read(cx);
+                    buffer_path_log_err(buffer).map(|p| p.to_path_buf())
+                }
+                Context::Directory(_) | Context::FetchedUrl(_) | Context::Thread(_) => None,
+            })
+            .collect()
+    }
+
+    pub fn thread_ids(&self) -> HashSet<ThreadId> {
+        self.threads.keys().cloned().collect()
+    }
 }
 
 pub enum FileInclusion {