assistant: Restructure ambient context in preparation for adding more (#11822)

This PR restructures the ambient context in the `assistant` crate to
make it more amenable to adding more kinds of ambient context.

Release Notes:

- N/A

crates/assistant/src/ambient_context/recent_buffers.rs

@@ -0,0 +1,25 @@
+use gpui::{Subscription, Task, WeakModel};
+use language::Buffer;
+
+pub struct RecentBuffersContext {
+    pub enabled: bool,
+    pub buffers: Vec<RecentBuffer>,
+    pub message: String,
+    pub pending_message: Option<Task<()>>,
+}
+
+pub struct RecentBuffer {
+    pub buffer: WeakModel<Buffer>,
+    pub _subscription: Subscription,
+}
+
+impl Default for RecentBuffersContext {
+    fn default() -> Self {
+        Self {
+            enabled: true,
+            buffers: Vec::new(),
+            message: String::new(),
+            pending_message: None,
+        }
+    }
+}